>>16940355As a Water-type trainer myself, I've learned a few things over the years as far as what Water types are really good on a mono-team.
Greninja stands out nowadays as one of the best Water types ever. There are all sorts of ways to run it, but the general consensus is a special attacking set. The moves you run generally depend on the rest of your team make-up, but Surf/Hydro Pump and Ice Beam are definitely must-haves. Since your whole team is water, Grass Knot is a powerful choice because it allows Greninja to resist both of Water's weaknesses and can hit other Water types supereffectively, if you need more anti-Water coverage (it OHKOs Mega Gyarados, for reference). Dark Pulse, HP Fire, and Extrasensory are all also good options. If you're focused on all special attacks, use Timid nature; if you decide to use U-Turn in one of your slots to get momentum, use Hasty instead. Life Orb is a solid item choice, but Expert Belt is a good secondary one if there's an item clause and you want a Life Orb elsewhere.
Gastrodon is another fantastic pokemon, and the set you use depends on what you need. Generally speaking, the only two moves you need are Scald and Recover, the other two moveslots are coverage/stall/whatever. I use a mixed tank set with Earthquake to assist my team in taking out Electric types; some people use Toxic to stall, I use Sludge Bomb to deal direct damage (and it has gotten me out of bad situations plenty of times). Leftovers is pretty much mandatory, but a Sitrus Berry might work too.
Vaporeon makes a good wall as well, and if you're using Gastrodon, you can focus entirely on special defense if you'd like. Wish passing is pretty much the best set there is: Scald, Wish, Protect, and something else. Roar is a dominant option for phazing out buff-oriented pokes, but Toxic is viable.
