It all depends on how 'magical' your underwater exploration is going to be.
Giant Air bubbles mean that you can speak and cast spells - within the bubble only? or outside too
Diving Bells could mean either - up to you
Underwater races can communicate - does this count as verbal components too (eg is a dolphins squeaks a language?)
Potions of Breath Water - no speaking = no spells
three directions to move in, strong currents (treat as rivers), darkness, water pressure, cold, hot, storms, fish