See above for previous on this subject.
In a nutshell, your maneuvering is hampered a great deal, depending on how much you are submerged. I tend to put that penalty on all maneuvers and attacks equal to the percentage of the PC is submerged.
So a PC walking in knee-high water might only get a -25 as they are 25% submerged. Up to their chest, maybe -75. Fully submerged would be -100.
This is of course countered if they have some skill or magic that allows free movement in such environments.
Also, this may be countered by Strength and Agility. But they are normally applied to all Melee attacks anyway, as part of the weapons skill. I would however apply any Strength/Agility bonus to any Static Maneuver made depending on what it is.