Snap - I also got hold of a SR4.7 earlier this year that had spent all it's life on an Oxfordshire reservoir and was covered in limescale.
The sailing instructors from the reservoir mentioned using brick cleaning acid but I was a bit concerned about the damage it may cause to the Hypalon and GRP.
I tried many different products but Starbrite Boat Bottom Cleaner worked best.
It is still hard work, I found the best way of doing it was to pour a small amount of the cleaner into a bowl and then using a small/medium size brush constantly work it in to the limescale agitating the brush all the time. Concentrated on a small area at a time, couple of hours per day for about a week but it removed all traces of the stuff. It took two bottles of Starbrite to get rid of it all.