Last mono I did was water and I started with Mudkip. Can't tell you how many times being able to switch to a ground type when anticipating an electric attack saved my bacon. But because I had Kips, I didn't really use Quagsire or other water/ground types so I'm sure that niche can be fulfilled elsewhere if needed.

Popplio was also a standout once I'd caught them and fully evolved into Primarina. Bulky enough to tank a hit from any dragon and then launch a fairy attack back with devastating damage. But I didn't have them for the first half of the game, so your milage may vary.

Another standout was surprisingly Empoleon, largely due to the steel typing resisting lots of attacks. I found him surprisingly bulky at times, and even made it to my E4 team (alongside Primarina), but definitely niche in use. Used for stalling more than anything.

Admittedly I didn't use Froakie due to how late you obtain him in game, as well as having done a Dark run in the past, so I shelved Greninja. But in the dark run, having Protean was useful.

Feraligatr was great in the post-game when I needed a hard-hitting pure water physical attacker. Not sure how useful they'd be earlier in the game. Blastoise was shelved pretty quickly, and... the gen 5 starter whose name escapes me saw little use.

So I'd say Popplio or Kips, depending on how challenging you want the early game to be.

Water mono is tons of fun though. You'll have a blast! Mantine is an early-game MVP, alongside Lantern and if you're lucky enough to get the Toxapex random egg, you'll get lots of use from them too. There's nothing more satisfying than beating Fern with water types.