The Lighthouse is a structure near the far side dock, it can be accessed by defeating the dock portal, and then pay 6 coins. It is made to allow safe passage to the island, although it can only survive a certain amount of days.
Once back to an island with a lighthouse, the boat will arrive unbroken at the far side dock. The disembarking builders will push then the boat to the central dock, near the Town Center.
The lighthouse can be upgraded to a stone one for 12 coins. The more upgrades it receives, the longer the structure will survive.