It has been decades since I worked in medicine, but from what I recall about scleroderma much of your physical activity limitations will depend upon the degree of the disease, and the progression rate. There are some effective treatments for mild to moderate cases of scleroderma, but the effectiveness of those treatments vary considerably between individual cases. If you are not in the progressive stage, and otherwise in reasonably good physical condition, wearing warm clothing, batter-powered gloves and socks, and a hat will all be helpful when the weather is relatively cool. When the weather is warm, be sure to protect your skin as much as possible by wearing long-sleeve shirts, long trousers, white cotton gloves and a wide-brimmed hat. Sunblock will help to some degree, but it's not as effective as the clothing.
I would also recommend taking someone with you that knows how to handle the boat singlehandedly. This is important. If you feel weak, tired or worn out, you can retire to the boat's cabin, relax. warm up and enjoy the remainder of the trip while your sailing partner mans the helm and sails.
Above all, don't take risks. Be sure to take the prescribed medications at the prescribed times--THIS IS VERY IMPORTANT! The medications will not cure the malady, but they can help manage it, and that's the best you can hope for.
Hope this helps,