It's approximately 1 atm per 30' of depth, so at 300' of depth, you are at approximately 11 atm (1 for the surface plus 10 for the depth). Or approximately 162 psi. (I checked this with Wolfram Alpha just to see, it says 9 atm / 133 psi not counting surface pressure, so 10 atm / 147 psi. Close! It must actually be 1 atm per 33' of depth.)
At a depth of 300', you basically have zero time at the bottom if you want to avoid the bends. Normal tables don't even list depths that deep. The "no decompression" limit for 150-190' of depth is 5 minutes at the bottom, on the US Navy tables (recreational diving tables are even more conservative). That doesn't mean you can't go there, but it means you can't safely return from that depth directly to the surface. You need to take your time with multiple stops on the way back to the surface, because your blood will have absorbed enough nitrogen that otherwise it will form bubbles in your blood vessels at surface pressure (that's the bends).
That said, if there are tanks intended for very deep dives, they probably would not be filled with plain old air. That will kill you. More likely something like a helium-oxygen mix (heliox) or something else with a reduced amount of nitrogen. So that mitigates it and would give you more time. This would also mitigate nitrogen narcosis. I'm not trained for dives this deep or heliox so I don't have any numbers for you. (If I remember correctly, we did a 105' dive during one of my advanced courses, on air, but that's by far my deepest.)
Also, at 11 atm, you are going through air really fast, because what you are breathing is all compressed by the depth. If you actually are just breathing out of a tank, it won't last very long. A rebreather would be more likely, which is recycling what you exhale, removing CO2, and then providing just enough of the appropriate gases to maintain breathable air. That would stretch out the air in the tanks.
Given that this is science fiction, I think you can safely play with the numbers however you want. What I would probably do, personally, is have rebreathers with a deep diving air mix, have a HUD in the mask which of course gives the status in dwarven and not in the PCs language, and pick a diving profile and submergence time that is dramatically interesting. Nitrogen narcosis is interesting (you can hallucinate...) so keep a chance of that. The chance could be increased by an equipment malfunction (could be due to a PC's error in initializing the gear, or due to damage). Give them enough time to do the thing they need to do and the necessary stops on the way back up, but only barely. Give them a problem at the end of the encounter (it's going to explode! release the kraken!) that makes them really want to just go to the surface immediately to escape, but the HUD is telling them DANGER DANGER DANGER MANDATORY STOP. Etc.
I'd not spelled it out precisely, but that's what I meant by the deep-dives tanks and the PCs having a suitable air supply, which would contain [some suitable mix of air] - the facility the PCs are in is actually where the dwarves had been conducting reasearch on the ruins the PCs are diving to, so the assumption is that there are air-tanks set up for their regular use (the dwarves had to evacuate the city only a short time ago).
The Aquatics skill check is much to work out how to turn the dwarf dive computers on (to get said mandatory warnings). I'm pretty sure at least one or two of the PCs speak Dwarven by the by (but it'll be the first time this party will have run across any).
Your numbers and skill checks seem fine to me.
Good to know, cheers!
Also, if the combat armor is a fully pressurized suit, inside the suit it is 1 atm. Narcosis is not an issue, air is being consumed at normal surface rates, the bends are not an issue. If the combat armor is only mitigating the pressure (maybe the pressure inside the combat armor is 3 atm rather than 11), because the suit is being squeezed, you could still have some of those issues but much less. (E.g. 3 atm is equivalent to 60' of depth, you can spend an hour at that depth breathing normal air and still safely return directly to the surface, and you won't suffer narcosis at that pressure either.)
I was working on the basis that (on checking) a spacesuit doesn't work as a diving suit. SM2 only gives armour as being "environementally sealed" which is not quite the same thing as "pressurised." I ahave assumed such armour suits are likely to be capable of functining in a vaccuum (a not uncommon hazard) but much less likely to be required to work in high pressure
1.
Sharr's suit I thought I would give the benefit of assuming it would be resistant to the pressures somewhat (maybe being rated to some level of pressures, but not that much; but I'm open to debate if you think it should be all or nothing. (He's a pilot by profession and the implication is he got this armour from a period where we was basically forced into becoming a pilot, so his armour is more likely to be more "space marine (not in the 40k sense) boarding armour" than "ground combat armour.")
If the combat armor suffers a serious breach, the inside will quickly go to ambient pressure, and the wearer is not equipped with a deep diving air mix or rebreather (unless the armor incorporates one, which is possible), so that gets ugly fast.
Oh, indeed. I haven't worried about that, though since (albiet the PCs have no idea) I'm not (currently) planning to actually have any combat while they're underwater (in a day quest, we only have time for one or two (fairly short) combats, and the one planned currently involve surface enemies). And if it does happen, the rules above (and the obvious drowning issues - wherein I might have to dig out the drowning criticals) should suffice.
Currently planning. But I haven't finished writing the quest yet!
Oh, also, everyone knows dwarves are more resistant to decompression sickness than other races. Since they spend all that time deep underground (which has the same issues if you go deep enough). So the computer in the rebreather is probably not going to get things exactly correct for other races anyway....
Actually, the dwarves of the Ziragthar Divinity who are by far the majority race of their worlds (most of them came from "fantasy" planets where the dwarves became the dominant speces (as opposed to humans) and everyone else faded into myth) live mostly on the surface nowadays. While they will still have a modest number who will live in the old sort of clan holds, the need for sewage, power, running water, information technology and the sheer size of the populations sort of mandates it. So that, at least, will not be the PCs major problem. (It's also worth noting that I'm pretty sure none of the PCs actually HAVE Aquatics, so they'll be making that skill check untrained; I didn't want to go overboard...!)
Some other random thoughts if you haven't considered these topics (I love underwater stuff)... At 300', it's going to be dark, regardless of surface conditions. (They also won't have any sense of surface conditions, e.g. waves won't affect anything at depth.) The PCs can carry lights but those lights won't illuminate very far. They'll be able to see things in color very close and in blue/green maybe 10's of feet. That makes it hard to find things unless there are beacons, markers, ropes, etc to work with. A PC who turns off their own light will easily be able to see the lights of the other PCs for a fair distance even if they can't make out any details (e.g. who is it?). They might even mistake the light of an adversary for an ally.
I hadn't gotten that far yet; but I think the last time I did and underwater adventure (for pure fantasy) I actually did some research and made a chart, though whether it goes to that depth, I'm not sure! I was intending doing a google image search (something that wasn't available the last time) to find some image taken at 100m as well, to give me some idea.
(They do have scanners though; multiscanners and Scanner analysis is as obiquitous as General Perception and Body developement: everyone has ranks in it, because the latter two skills are easily the most used in the entire game. Once they identify which bit of the seabed is actually the ruins and not industrial mining operations (easily done if they swim out from the facility, because the dwarves had cleared a path through the seabed in preparation to build and underwater tunnel and dome to cover the ruins), scanners will assist in searching. On the flip-side - they actually don't know what they're looking for (and thus what to scan for!))
But it's worth mentioning, so I can make sure I check what light-sources they have!
Communication can be a challenge underwater. A standard mask and breathing gear doesn't permit speech. Even if you take a regulator out of your mouth, you can't talk because the air bubbles coming out of your mouth overwhelm the speech sounds you are trying to make. You need a full face mask to permit speech. If you feel generous, the diving gear could incorporate that. A pressurized suit gives that benefit as well. But it might be more fun to make the players use hand gestures.
Radio is basically worthless underwater. So if the PCs are used to relying on radios for communication, that won't work. Again, you could provide suitable communicators (probably ultrasonic, or gravitic or neutrino or whatever ultratech is normal, I don't know) if you want, but it might be more fun not to.
It'll depend what exactly the PCs use. Their communicators are way more advanced than radios, but for anyone in scuba gear, it won't be any use! That said, they do have a natural telepath in the party (and we're using Sci-fire alternate rules - that is, psionics as skills not spells), so that probably not going to be an issue.
Probably.But, again that is an aspect I'd not thought about yet, so I'll keep a sharp eye on exactly what I will let them communicate to each other, depending on how they gear themselves up.
Thanks for all the suggestions; I'll make sure to take them all on board.
1As the vast majority of races around the galaxy breath the same atmosphere (thanks to copious Sienetic Harbinger Probability Engineering, retroactively modifiying probabilites of planetary formation such that there are dozens of what is most easily described as "fantasy" planets, as well as outright direct copies of planets (there are multiple Earths and their surround sectors of space, for example), the need to wage war outside of terrestrial environments is not high and so it would follow that the average, run-of-the-mill nonspecialised battle armour would not need to be rated to a particularly wide range of atmospheric pressures.