A Triton is entirely appropriate for a maritime campaign.
What environmental challenges?
Water breathing is literally a 3rd level spell (with the Ritual tag) that lasts all day long, doesn't use a slot and is available on the Druid, Ranger, Sorcerer and Wizard list. Plus any Druid who can literally wild-shape into a fish.
The guy can swim and breath water early. Good for him. For my campaigns, that would be encouraged. It would be a feature of the campaign and not a bug.
Like; I actively encourage players to play characters that will shine in my adventures. Undead heavy campaign and you want to play an Undead slayer Cleric Paladin? Please do!
Problem is he's only 1 PC.
Water breathing doesn't have a swim speed and can be dispelled.
So his racial features are either useless or he can effectively fly and outshine everyone else.
In the one game I've played he seems a bit more all about me than the team. Eg he was suggesting people do stuff to help him out which undermines the ranged characters.
And yeah bad experiences in previous games when someone has significant mobility advantages over everyone else.
He's also rolled really high stats and the Tritons 3 +1 may as well be be 3 +2's.
He was directly told to not deliberately build a bad character but not to min/max to much at the other end.
His stats would be something like 14, 1616,18 and wants to marry up class from Xanathars, MC it and have a race that gets around a few challenges I've got planned.
He'll be good at range, melee, key everything off charisma, and have a build that shuts down a lot of things.
I can see others getting frustrated with that.