Seafarer's Handbook doesn't mention anything regarding the use of Cold spells underwater, except for Ice Storm which it says deals cold damage normally, but less bludgeoning damage. In my aquatic campaign I just adjudicate spells on a case by case basis, generally changing the visual effect slightly (fire turns to steam and boiling water, for example) but leaving the spell effect as described. Too much hassle otherwise. For cold spells, I'd just deal cold damage normally, freezing the water briefly, but so quickly that it can't form solid ice. Or, if you want, anything in the target area freezes internally, so the water is not affected at all.
Pinotage
Pinotage
