A further complication would be that spellcasters without waterbreathing are particularly scrooged... casting a spell with Verbal components uses most or all of your remaining breath (maybe a verbal spell costs 10 breath points instead of 1).
On a parallel note, what's your collective takes on...
As I'm running a campaign that will soon involve the party going a minor distance underwater, without an easy source of water breathing, I'm wondering how you folks feel about the standing D&D rules for holding breath, drowning, and the like. As it stands, a character can hold its breath for a...
Shaaladel and Shalosha join the party, along with humanoid interpretations of Trilla and Naizelasa (not that the latter would ever deign to take a lesser form, but you know, roll a Nat20 on Persuasion, and all bets are off)...