Why? It is like Suicune pretty much lol. Tail Glow can't set up on offensive much to be honest (due to hard hitters like Latias for example), and stall teams have Toxic Spikes, TrickScarf, Perish Song, etc. Then there's Rest, which requires Rain Dance support, and also forces moveslot syndrome lol. Surf / Ice Beam is walled by Vaporeon, bulky Waters, etc, Surf / Energy Ball is walled by Latias and the like, and Surf / Ice Beam / Energy Ball / Rest is dumb. Calm Mind / Surf / Rain Dance / Rest w/ an EV spread like 252 HP / 224 Def / 32 Speed is my favorite set (of course with support like Toxic Spikes for Vaporeon).
But, yeah on paper it looks really good, but in general all it is is just another Water-type.