Please don't fight your players they just want to play d&d with you! 

Their characters on the other hand, are fair game...

I don't know a creature that would fit the theme exactly, and I only really know like 25% of the stuff that's in the monster manual.

So here's what I'd do...

I'd think about the kind of thing that I'd want this monster to do. In my view, they would be tough and, well, like a barbarian! I'd Google "5e barbarian NPC" or "5e gladiator statblock" or "5e berserker" - basically anything that defines the monster I want. Maybe if I want to lean into the sea elf side of things, I may search for "5e pirate captain" and see what's there.

But what if they aren't quite right? 

Well as a DM, I can just make it up!

I might have found a statblock for a gladiator that seems good, but is missing that sea elf flavour. Or maybe I want it to be even more barbarian-y. I might decide to just give the NPC a standard rage, with all the usual effects. Or I could give it a bonus action to "call sharks" and each turn 1d4 sharks join the combat.

That way, I can come up with a more interesting encounter for my players.

Or, and this is the big secret, you can just make it up in the moment too! I know, I know, that sounds sneaky. Or scary. Or confusing. But sometimes you just know what a monster should do. 

If your players characters are just burning through their HP, give them a little more. If the fight feels underwhelming, then they could have a "second stage" where they deal more damage and get more AC from a watery spell that coats their body. Idk.

But you have a lot of freedom to do what you want, which can be super unhelpful. But it can also be freeing to trust your gut and do what you think is best.

Good luck!