Move with back and forth with W and S (left stick)

Look/steer with mouse (right stick)

Shoot spear gun click (x xbox, square playstation)

Interact with purchases with E (Y xbox, Triangle playstation), Move purchases left/right with A and D (left stick)

Jump out of boat with C (B xbox, O Playstation)

You have to be in your little boat to sell things at the merchant (bigger) boat and buy things at the shop.