Quickleaf
Legend
Mostly going the "mythic" route I see though there are a few "use allied resources" entries and the Rogue's player is using player actions rather than in-character assets. That pretty much matches my earlier list.
Huh, we must have different definitions of mythic. Maybe near-mythic for some of them... let's see of the 12 examples I gave (and this is high-level D&D we are talking, so presumably at least 11th level or up), I'd say 2 strike me as mythic and I'm not entirely happy with them, the paladin & the fighter examples for surviving underwater.
For the paladin it actually is something that kind of makes sense, since great saints were known to live without food and water for incredibly long periods of time, and there are mystics who could slow their breathing down immensely. Back that with the power of D&D faith, and ok, I can buy Sustained by Faith. Using it to venture underwater flies in the face of what I normally associate with a paladin doing, but it also could be used to, say, survive torture while imprisoned by the blackguard king who betrayed you - and that feels like a paladin!
Now, the fighter example is totally mythic, I'll grant you that. If we were going to make it more realistic I would give the fighter with the right talent (and performing proper preparation exercises) a maximum breath hold time of 20 + their Constiution modifier minutes; after that they begin drowning. Stig Severinsen, a German free diver, held his breath for 22 minutes back in 2012, so let's hold that as the limit of human capability. Of course, he didn't move at all in those 22 minutes, but let's assume the high-level fighter with the appropriate talent can do what Stig Severinsen did while fighting underwater and getting stabbed! It's a little hard to believe, but we can imagine it withing the realm of possibility for a character who is repeatedly cut, bludgeoned, impaled, poisoned, chewed on, fried with magic, and otherwise damaged as part of their daily career.
So compared to the 3e water breathing spell which lasts 2 hours per level, this may seem insignificant. However, if the system limited the water breathing spell in a way that 3e does not, there would be some parity there. As long as the adventure only involved a brief bit of exploration and a fight or two underwater, the fighter could hold his own.
An alternative would be to add a Lady of the Lake type figure which a fighter could take a talent for. Personally, I love this idea, though it may be more assumed setting in rules than some GMs want.