Boats get a fair share of abuse from minerals in lake or pond water and from salt in sea water. The minerals and salt form a lime scale that runs around the boat's hull. The lime scale is caused by oxidation of the minerals from the water. Lime sticks to the boat like glue and is not the easiest thing to remove, but with proper cleaning solvents you can remove the ugly water stains and restore your boat's hull back to a shiny luster. Toilet bowl cleaners contain ingredients that fight hard water deposits.
Wet the hull of the boat with water from a garden hose.
Fill a spray bottle with a toilet bowl cleaner, leaving a 2-inch space from the top. Fill the bottle the rest of the way with water.
Spray the toilet bowl solution onto the water line on the boat hull. Allow the solution to sit on the stain for five minutes.
Scrub the stain with a sponge or nylon brush. Avoid using steel wool pads on the boat hull as you may damage the gel finish on the fiberglass.
Rinse the boat hull with the garden hose. Make sure to remove all the toilet cleaner from the boat's surface. Dry the boat hull with old towels or allow to air dry.
Apply a boat wax to the hull of the boat. Place a small amount of wax on a soft, clean rag and apply to the boat hull in small circular motions.
Wipe the wax from the boat after the wax has turned into a white haze. Wipe with a soft cloth, making sure to remove all wax from the boat. The wax protects the gel coating on the fiberglass from mineral deposits in the water.