I don't seem to have seen this in such a simple form: I want a plugin that simply removes the whole "drowning" aspect to Minecraft. I remember looking a while back and not seeing anything that simply eliminated it (though some did when wearing special helmets).
Not 100% sure, but I think WorldGuard has an option in the configuration file to change if breathing underwater is possible.
Here you go: <Edit by Moderator: Redacted mediafire url> Its a simple plugin. It just stops all damage taken from drowning.