After collecting all minikits for a level you're able to free the ship from its bottle in the dock area of the hub (found by going all the way to the right) and hop on board to sail through the harbor. Do that with all of them to unlock this.
Although it is more time consuming, I suggest you don't free them all at once but just one at a time. Jump on it quickly and sail it a short distance and then jump off and move on to the next. There are 20 ships total.
If you need help finding any of the minikits there is a handy walkthrough that is sorted by chapter and can be found HERE. Just scroll down to the Freeplay section.