A boat bottom scraper is a tool used to remove barnacles, algae, and other marine growth from the hull of a boat. It is also known as a REEF, which is a four-letter word that is one of the solutions to the crossword. Boat bottom scrapers are typically made of metal or plastic and come in various shapes and sizes. They are used to scrape away the growth from the bottom of the boat, allowing it to move more efficiently through the water.