Already answered by PeterHolmes74
Some excellent water type megas include Mega Milotic, which requires tesseract, Mega Feraligatr, which you need dive for (can get BEFORE kepler city gym leader), Mega Blastoise (if purity mode), and Mega Swampert, whose mega stone is in Narra Town (not great since you don’t have drizzle politoed until post game, but still bulky). Gyarados is always a great mon, could go Cloyster or Azumarill for early game if ya want. Finneon has a particular use for learning Rain Dance early on, for the Orion gym battle. Tentacruel is really good as a special tank, who also learns toxic spikes. Grabbing Gastrodon/Swampert/Seismitoad before Xavier is not a bad idea at all, considering Water type’s weakness to electric, giving you one less type to worry about. Lanturn with Volt Absorb is also excellent for a good portion of the game, giving you a mon that can damage East’s Skarmory and being quite effective against Audrey1 fight (apart from their own Lanturn). Delta Haxorus is great in its own right, naturally learning priority moves, being a Steel type, and thus immune to Toxic, and getting Swords Dance. Water types also get good Shell Smash users in Cloyster and Barbaracle, if you want to set up and sweep. Delta Liligant (water) is also nice for lategame, if ya need that. For e4, was thinking…
Milotic (mega)/Delta Haxorus/Gyarados/Gastrodon (or any of the other water/grounds)/Cloyster/Lanturn
However, you can also make several variations of this and be generally fine. Like swapping out Lanturn for Delta Liligant (water) to help the matchups against the many Scizors you would face in e4.
- You can dexnav surfing or fishing rod mons in the grass and dexnav fishing rod mons by surfing, if that is the answer you desire. Still requires the mon to be caught of course.