You'll have to breed to get Rock Blast, to do this get a female Shellder/Cloyster and breed it with an: Omanyte, Omastar, Corsola, Anorith or Armaldo, that knows Rock Blast.
Take the Shellder that hatches (which should know Rock Blast) and level it up, it then learns Icicle Spear at level 13, Ice Shard at level 28 and Shell Smash at level 56.
Then finally, evolve it into a Cloyster with a Water Stone.
Hope that helped.