Abstract: A boat enclosure system for performing boat maintenance work on the water has a U-shaped floating dock forming a three sided enclosure with an opening at one end to enable boats to sail into and out of the enclosure. A bag of flexible, waterproof material is suspended around part of its periphery from the periphery of the dock to form a chamber below the dock, the remainder of the bag periphery extending across the open end of the dock so that it can be dropped into a lowered position allowing clearance below the water for boats to sail in and out of the chamber, and pulled up above the water level when a boat is in the dock to form a closed chamber below the water surface around the hull of the boat. The chamber is connected to a pump and filter unit for circulating water from the chamber through a filter and returning filtered water back to the chamber during maintenance work.