Holding your finger down on the screen will cause your character to move forward on the wave. Releasing your finger will cause your character to back off.
Be sure to change your height on the wave as needed in order to avoid the seagulls and to go for as long as possible.
When you see an offer to get twenty coins if you watch a video take it.
Once you collect two hundred coins you'll be able to unlock a new character. Don't have high hopes regarding it because nothing really changes ie gameplay stays the same no matter which character you choose.