Before use of any of the below products, methods, or water spot removers make sure to read the directions and first test the cleaner on a small area of the hull. It is usually recommended that products be applied in a shady or cool area, as opposed to being in the sun. Some methods also work for calcium build-up on an outboard engine and lower units that are exposed to heat exhaust. Avoid cleaning your boat in the water to reduce the risk of toxic materials spilling into the water.
Try the Following Water Spot Removal Methods:
1) BABE'S Spot Solver
BABE'S Spot Solver is specifically developed to remove hard water spots caused from mineral build-up. BABE'S Spot Solver is a non-abrasive solution designed for all gel-coat, glass and chrome surfaces.
2) Vinegar and Water (homemade)
Mix one gallon of water with one cup of vinegar. Rub spots with a soft cloth. A few applications may be required before spots are removed. Note that after this use your wax will most likely be gone. Don't forget to re-wax.
I've heard that there is an actual product on the market called Cleaning Vinegar that works really well.
3) Lime Away / CLR
Though they are fairly aggressive products, CLR and Lime Away will also work, and many swear by them. Use these for the more severe cases of neglect.
To make your own, mix one part Lime Away and 10 parts demineralized water. It works best while boat is still wet after use. Spray it on and wipe with a soft towel. Don't let it sit for a prolonged time. Rinse away immediately. Follow with a hull cleaner and wax.
4) Isso Fiberglass Reconditioner
Restores color to faded fiberglass. Removes oxidation and chalking, water-line, water spots, tars and oils.
5) Other Household Cleaning Products
Some use shower cleaning products such as Shower Clean and Lysol. Like most other methods, these products work best when the hull is still wet.
6) Extra Tidbit
For a yellow stained hull, lubricate it with WD-40 spray.
MORE HELPFUL LINKS:
Vessel Cleaning: Alternatives to Toxic Products - from Nautical Know How, Inc. Includes many household suggested items.
Vertglas Gel-coat Restoration System - Restore like-new shine to fiberglass and gel-coat. From Marinestore.com.
Teak Cleaning Tips - from Boat Tech/Boat U.S.
Fiberglass Repair - from Boat Tech/Boat U.S.
Gel-coat Scratch Repair - Gel-coat choices, color matching, preparing the scratch, catalyzing, spreading gel-coat paste, covering the repair, and sanding and polishing. From Boat U.S. Don Casey shows you how.
Restoring the Shine to Fiberglass - Cleaning, degreasing, wax, polishing, using rubbing compound, electric buffer, and restorer information. From Boat U.S. Don Casey shows you how.
Teak Care - Cleaning, oil, sealers, and varnish information. From Boat U.S. Don Casey shows you how.
What is oxidation and how to get rid of oxidation - scroll down this linked page for an answer. "Oxidation is the result of your gel coat reacting with oxygen and sunlight. Oxidized gel coat appears as dullness, cloudiness or fading in the color of the gel coat and is typically rough to the touch. You can't make your boat shiny unless you remove the oxidation." Marinestore.com