There is an area to the right of the main level selection hub, with four piers. After you find all 10 minikits in a given level, a bottle will be hanging on a corresponding pier. Break the bottle, and a ship will appear.
To get the trophy, you need to sail all those ships - get all the minikits, break the bottle, get in the water and press
to sail away. You have to sail every one of those ships to get the trophy.