Wait, but why would you need the lapras JUST for fighting other water pokemon? :o? If you have magneton then that's kind of useless. I'd go with the rain dance/thunder so you still can pack an even better STAB Surf :)
I guess i should start my Platinum Run by the way.
no, Lapras is a pokemon i want to be able to use against anything, that's the point. with that move set, it has two strong STAB moves and then another move to help it fight the only pokemon type it has trouble doing damage to (other water types), making it an offensive weapon while it's also a tank that's hard to bring down.