At worst, I'd give a % chance to fail for somatic components, not not require Freedom of Movement. We are not talking about running, just making jestures. The only one water would really interfere with would be a broad flailing of the arm, and that can't be vital to evey spell in the book. Now spell books and components exsposed to water could be ugly.
BTW, ditto on the Water Breathing, or an Amulet of Adaptation. You really can't get around needing to breath to speak, unless you really want to complicate things by letting every spell take off of the amount of time you can hold your breath.