>>23134866I'm sure Specs and Assault Vest will probably be most common sets at first. It has more Sp.Atk than Keldeo (just), with a Hydro Pump that has better accuracy and a 30% burn rate. So Specs is a bit of a no-brainer, though the Speed is very lacking when compared to Keldeo of course. But force a switch and smack things hard. There's not many things in the game that actually enjoy the 12% residual damage from burn, especially if they have to take a Hydro Pump from base 130 Sp.Atk backed by Specs at the same time.
I think this
>>23134822 anon is right, in that it'll be more niche version of Heatran. But I feel pretty certain it will be mid to low OU. I can't see it dropping to UU. It of course has its downfalls - mediocre Speed with no priority, SR weakness meaning it's easy to wear down, its weaknesses are pretty common (Rock, Ground, Electric).
But it has a lot of good qualities at the same time that more than balance those out (Water immune, Hydro Pump + Scald signature move, great bulk and attacking stats, great coverage options, not weak to Water and Fighting like Heatran, nice resists to Steel, Fairy, Fire, Bug and Ice etc). Seems like OU material to me.