Please use great caution if you plan to work under a boat on a home-made gantry/lift of any kind! Also, I would hesitate (that means I would not do it) to put water into the hull while it is suspended. There are any number of Youtube videos of boats falling because straps slipped. You (and I ) can quickly forget how heavy that added water might be. Overload of your lift could happen too quickly. Please be careful.
