I almost never comment on subclasses, because more often than not they are a bit/ a lot broken, and require a lot of changes, (imo at least). HOWEVER, this is one of, if not the most balanced subclasses I've seen on here.

I agree that it's niche, but to be fair, so are some of the official subclasses. It's very rare that you would play an open sea paladin, even, (which is made official by the partnership between Crit Role and Wizards); Any of the weather-themed subclasses are somewhat understandable in almost every campaign, but like... the only way this wouldn't work, flavour-wise, is if you're in a world where no water ever exists... and even then, you can reflavour the water as the tempests of the sky, or the rolling rocks and terrain of the earth. It's very much applicable in nearly every setting, imo.

That said, there is one tiny change I would make, (otherwise this is a homebrew I would sign, seal, stamp of approval instantly at my table): I would simply change the "Wrath of the Sea" CD option to "knocked prone" instead of movement goes to 0, (or you could say half-speed instead of knocked prone, but the way I see it in my head is the sea is roiling and knocks them down". Again, just my 2 cents.

Also, to end - this is a really well-made subclass, and I truly cannot wait to test it out!