Except the rules are written in such way that they render holding breat irrelevant. You may as well write "unless in combat a character can hold their breath. When in combat, you must roll concentration at end of your turn or suffer level of exhaustion. DM may decide to treat particularly dangerous or prolonged situation as combat at their discression". And done, you didn't need to invent new rules just for it, you used an existing system. You could even simplyfy it further and just slap it under concentration rules.