Change Walrein for a Kingdra. Best water Pokémon ever since his only weakness is Dragon. It has good stats and a good move pool. His Swift Swim ability is awesome too.
Teach him Rain Dance and together with his ability it will be very fast and it raises his Water attacks too. Teach him Ice Beam or Dragon Pulse to deal with Dragons (his weakness) and teach him a Water move off course for the Rain Dance. As a fourth move you could give him Body Slam (very good and it may paralyze the foe) or Smokescreen/Double Team.
I hope this helped :D
Don't mean to knock Kingdra, but it's also weak against fairy types which are pretty common. If you add Kingra then 4/6 of your Pokemon will be weak to fairy. Just keep that in mind.