well, if you opt not to evolve mudkip before lv.24, it learns mud sport, and is weak to electric attacks, Relicanth is weak to it, and learns it...as does a ton of water types weak to electric attacks:
http://www.psypokes.com/dex/techdex.php?id=300 click egg moves for the list...
As for water sport, I say surskit/masquerain...
PLUS, those moves can be baton passed on...helpful if you have pokemon weak to moves in your party.