And hands(gloves help). All these parts just don't take the same amount of abuse. Luckily many of these problems can be minimized by planning differently, pacing things out (knee pads and cushions on hand).

Creative use of staging around a boat can make all the difference. I learned on houses, if you can get yourself in a comfortable working position, and then put your work where your hands are(via staging), you can enjoy your work, safely, instead of enduring it.

Not always possible with a boat(a lift would be nice...), but there are a few tricks and the alternative, ladders, can be dangerous if misused. Working in an uncomfortable position(you pay attention to pain) can lay you up in no time. Some flexing is good and will help you feel better as you do it.

It's all a challenge when you're still lucky enough to physically do it.