Summary: Will this replace Swampert in OU? No. But there are LOTS of Swamperts on this forum, and 0 Gastrodons.
When switching into Boiling Water, Gastrodon > Swampert in 3 ways - (a) takes no damage, (b) raises sp. attack, and (c) Swampert can be burned into uselessness - burn doesn't do jack squat to this.
IVS: 31/31/31/31/31/31 - Flawless. You MIGHT want to opt for a lesser Hidden Power IV set for coverage here, but with Recover/EarthPower/IceBeam OR Blizzard/BoilingWater OR Muddy Water OR Surf, it's movepool is pretty much determined. Replacing Recover with, say Hidden Power... grass... doesn't seem worth settling for less than flawless IVs.
PS - I've worked myself up into a frenzy - if you pass on this, please let me know and I'll approach MeGaMaN X or ObviousEnd or another RNGer about it.