[Grellan]

there are three useful and relatively low level magic defense spells that your bishops should have by level 15 or so; Soul Shield, Magic Screen and Element Shield which are all 4th level spells.

Magic screen can be cast outisde of battle so you should have it up once you get into the water. this is a general anti-magic spell.

Soul Shield and Element Sield can only be cast in combat so you should try to get them up in the the first round. soul shield will protect the party from the psionic attacks of things like psi sharks. the rays either cast psionic spells or water realm spells (can't remember) which is where the element shield protection comes in handy. i seem to remember other rays which didn't cast spells but relied heavily on missile attacks so having missile shield up in advance (it too can be cast outside of combat) might also help.

more generally, if you have saved recently before going into the watery sections then you could try playing from the reload. the random monsters vary each time you enter the new area so you could try saving just before entering the water section and then hoping for a less tough group.