Actually... Piplup uses Surf in brawl........ o.o take a look at that. Piplup makes a small wave appear out of thin air in front of it and sort of just rams into its opponents with the water or something. You could make it work like that. Better yet, you could make that how Lapras moves around!
Hmm... well you don't really need a move to know how to swim in the water do ya? :p I'm sure Lapras could do that naturally without the move. In reality, using Surf on Land would be kinda worthless. I'm sure it wouldn't really matter if you still used water transportation without Surf.
Alot of silly things like that in Pokemon. Like, you can't cut this thin little tree in the way unless you learn cut, NOT SLASH which is way better, no it has to be cut and they can't just use their own claws they were born with to cut down the thinnest tree in existence. See where I'm getting at? :p
If you used waterfall, it would still be a good attack and you could still travel along water naturally. Lapras won't forget to swim or anything silly like that XD