T’was something we ended up houseruling but you are on the right track.
Swimming ended up being based on a kind of D&Dish Difficult Terrain converting to one 1/2 or even 1/4 of their movement that they “spent” on their turn.
This increased the Activity % the PC/NPC spent in a round and, thus, increased the penalty amount to do anything else in the same round.
For more crunch, we allowed someone to modify this with a 1% bump for every +1 Total ST mod they possessed.
So someone with a base speed of 50’ could wade into water at 1/2 of this speed as a total(up to 25’/rnd), but costing the same %(effectively double for what one is getting out of the movement.
Fully submerged was 1/4.
A +10 ST total bonus meant they could eke out 10% more total swimming speed(2.5’ from the above example).
We then just used a combat mod rule already in-game.
As with D&D for a base, Thrusting weapons weren’t penalized but Swinging weapons were(at -40 for RM’s ‘Full “Soft” Cover’, it was decided since water was technically in the way!).
Hope my quickie dirty rule helps you create your own until someone can point to canon rules, I am unaware of.
