Rocksmash can be found on Route 36. After you battle the Sudowoodo there will be a man to the right of the tree and he'll give you Rocksmash. After that you can buy it in Goldenrod City.
And to get on the SS Aqua you got to beat the Elite Four talk to Professor Elm and he will give you a ticket for it (it only runs on Mondays and Fridays).
If you got anymore questtions then please post them in the Simple Questions in the right forum. *Closed*