Physical: Earthquake/Iron Tail, Waterall, Encore, Avalanche.
Special: Surf, Hail, Blizzard, Encore.
Physical Explanation: Earthquake and Iron Tail are both high damaging moves, so choose which one you like better. It depends on whether or not you need the additional efect of Iron Tail. Waterfall is the best physical Water type move in the game. Then, I chose Encore for some stalling, allowing you to be able to beat opponents who decide to use stat moves. There is Avalanche due to the fact that it is Physical Ice, and Walrein is slow.
Special Explanation: Like before, Encore is a useful move to help Walrein survive. Surf is the best Water type move, so that is a must. Hail not only makes Blizzard have 100% accuracy and a 25% chance of breaking through Protect and Detect, it also damages the foe and heals a Walrein with the ability Ice Body. Blizzard is there to work with Hail.
Also note that Sheer Cold should not be used seeing how it has low accuracy and is banned from competitive play.