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This came up recently in a game I was running. I handled it with a decision at the table. After more thought, this is what I came up with.
First, I don’t allow you to cast a spell that you mumble under your breath. In air, You must say the magic words (the verbal component) in a clear and forceful voice that can be heard from at least 20 feet away.
Here are my house rules regarding speaking and casting spells with a verbal component while you are underwater.
1) After 1+(con bonus) minutes of holding your breath underwater you begin to drown. Each round that you speak or attempt to cast a spell with a verbal component takes 30 seconds from the time you can continue holding your breath. If you are just talking, this can be a simple sentence no more than about 10 words.
2) You are harder to understand when you talk while underwater. There is a 50% chance that you won’t be understood when speaking, and a 50% chance that your spell will fail when uttering the verbal component.
3) Sound travels further underwater, so she verbal component of a spell will be heard at least 40 feet away. The same for normal speech. If it can be understood, anyone further than 40 feet away will have to succeed in a perception check with a DC = the number of feet beyond 40 feet.
4) You can’t whisper or yell underwater.
5) If you are underwater, no one that is not in the water will be able to understand anything that you are saying.
6) If you can breath underwater you can talk and cast spells without restriction.
You may also want to see to my post regarding drowning here.