Usually, DMs will allow players to speak underwater with a water-breathing effect. This is because the rules as written state the only thing stopping characters from conversing or using verbal components underwater is their need to breathe, so without this need, they should be able to speak.
This does of course mean before this effect is in place, spellcasting will with verbal components not be possible unless the spellcaster has meta magic such as the Sorcerer's Silent Spell.