Uh, actually Waterfall is a very decent move in-game and competitively for Samurott, considering it's STAB, 30% flinch chance, and Samurott's base Attack of 100. Samurott also doesn't have many other options for its moveset, since Aqua Tail is 90% accuracy, X-Scissor you don't get until late in the game, and Megahorn requires a Heart Scale and has low accuracy. Surf is pretty much the only other stable STAB it gets.