Tbh, there really is very little flexibility in the set we're going to use, Water Arceus will need to run Recover, Grass Knot, Status, Roar / Judgment or Surf. For this reason we're going to skip over the set voting stage and just head straight into the next team mate stage.
I know this may seem a little silly, but hear me out, I propose Xatu. Xatu, as one of the two usable Magic Mirror pokemon has better defensive stats, better typing, and access to U-turn. Why Xatu?
Well, as a team with no hazards and little immediate offensive presence from our weather summoner we NEED a way to repel the countless hazards that are going to litter our side of the field. Xatu can also provide nice dual screens support and switch into Ferrothorn all day.
What say ye, ubers?