Soaker Cola
Ultra rare0.1%GrindOptionalAnywhere
DLC Surf'n'turf
Wash 150 dirty plates or glasses with the water gun
On some levels there's no sink - instead you must grab the water gun and squirt them instead. A lot. But its fun. You can replay level 2-1 to grind this out if needed.
As with many similar achievements in this game, you must be player 1 for this to accumulate and unlock.