I think one word sums this up:
WOAH.
Gastrodon East Sea's Osmotic Pressure + West Sea's Raging Flood = Raging Flood is unbelievably strong. Plus, Wild Waves helps to put a couple of damage counters onto East Sea. It also helps East Sea's Dwindling Wave massively.
Weakness is average, Resistance is alright, Retreat Cost isn't good. If you run this, best to run Switch (yes, really) or Super Scoop Up (not recommended).
Ideally, you need to run both of them, otherwise there's no point. East Sea is better for KOing Infernape and other annoyances such as Magmortar and Typhlosion, and West Sea also attacks tricky Lightning-types.
East Sea and West Sea both get 8/10 on this one, methinks.
(Yes, I used the same review on two cards, so sue me. :P)