Klay Thompson -- Underwater Twerk Fest ... At Vegas Pool Rager

It pays to be #1 ... just ask Golden State Warriors superstar Klay Thompson -- who celebrated his NBA championship with a 2-on-1 underwater twerk session at a Vegas pool party.

It all went down at the Liquid Pool Lounge party at the Aria ... where Thompson and a bunch of his teammates let loose ... poppin' champagne and dancing with hot ladies.

But the highlight ... when Klay and two female friends hit the pool and got their grind on!

Afterward, Klay made a video with two OTHER ladies ... while slamming Moet.

Wonder what the Cavs are up to?