jb. Vinegar works just fine to clear any calcium deposits.
However If you want the nuclear option then Hydrochloric acid is the one to use.
YOU MUST ONLY use it if you are familiar with the safe handling of chemicals and their effect.
YOU MUST ENSURE you have some alkali ie lime to nuetralize it in case of spills and plenty of water
YOU MUST WEAR SAFETY CLOTHING
AND TAKE EXTREME CARE.
For GRP hulls, using protective clothing
and goggles 'wet spray' a 10' section of the hull
, let the spray work for 3/4 minutes, scrub the wetted area with a nylon brush and rinse off with copious amounts of water, then repeat for the next section, it will attack any metal/mild steel
so avoid through hull
fittings, props etc, it will clean discolouration and leave your hull pristine white, wash the hull with normal detergent, dry off, and it's ready for applying a good polish, it will also remove barnacles
It quickly does a fantastic job providing you use it sensibly and take proper precautions.