I have a 1960s Montgomery Ward Sea King Boat that has been turned into a pond. The fiberglass hull has been sealed inside and out with two layers of Flew Seal. The flooring has been redone with vinyl wood flooring for a nice look whether being used as a pond or a boat. The pond has a drainage system to keep water in and does hold fine when fish are added to it or pull the plug to drain it. Trailer comes with the boat/pond, neither have titles as owner never got around to it. If you want to use it as a boat then slap a motor on the back and she will float. All pond accessories and decorations come with it.
Any additional customizations will cost more and a down payment.