The beach is an apt place for a mk3!
Collinite waxes are used in the marine industry. I would suggest using these as they provide very good protection and aren't too expensive.
I would try either 876 insulator wax or 845 paste wax. I would prep the car with a clay and polish before you wax it, otherwise you are sealing all the bonded contaminates in.
I would consider parking your car away from the pier where it is getting wrecked... heavy sea spray contains stones and salt which will ruin your car over time.
Wash your car using Bilt Hamber autowash... not only is this a VERY economical shampoo but it contains rust inhibitors so when it comes into contact with bare metalwork/stonechips it won't encourage corrosion.
Do not just brush the sand off as it will scratch your paintwork, especially if you haven't protected it. This includes your windscreen as glass is composed of sand
