I've been doing an Italian playthrough of Ultra Moon for fun, and I decided to give myself a little challenge by using an all Water-type team. However, I came to a screeching halt when I tried to decide on my last team member. Here's my team planned so far:
Primarina - My starter. It can use some pretty powerful moves and has always been useful in my playthroughs. With Sparkling Aria (for its Z-Move), Moonblast, Psychic, and Energy Ball or Ice Beam, it can pack quite a punch.
Pelipper - Drizzle will activate Rain when it enters battle, boosting the power of Water-type moves and giving Hurricane perfect accuracy. This will come in handy, as my entire team is at least part Water type.
Toxapex - Quite bulky, which can come in handy in multiple ways, such as for reviving or healing other Pokemon with items. Though usually hyper offense is the way to go in-game, I've found bulky Pokémon quite useful.
Lanturn - With Volt Absorb, it's immune to Electric-type moves, a weakness among most Water-types. This makes it quite useful and it doesn't posses the 4x Grass weakness that Water/Ground-types have.
Vaporeon - It has decent special bulk, but with a lack of quick recovery (it gets Aqua Ring and Wish, but Wish is only through breeding), it's not the best. I'm considering replacing it with a different Pokémon, though I adore it.
I'm completely clueless, so any help is greatly appreciated!