Hold on there - this is Wailord we're talking about, right?
Simply, without cheating (Sharking), then there is no natural way of teaching it Body Slam, as far as I know.
The moves it learns naturally are...
Splash, Growl, Water Gun, Rollout, Whirlpool, Astonish, Water Pulse, Mist, Rest, Water Spout, Amnesia, and Hydro Pump.
The moves you can breed in are...
Double Edge, Thrash, Swagger, Snore, Sleep Talk, Curse, Fissure, and Tickle...
Normally, I'd have said use a Heart Scale to get the move back, if you'd gone past it (like Medicham with the elemental punches), but it looks like RuSa Wailord with Body Slam is pretty much impossible, except for possibly promos, or whatever...