Re: Is it safe to work on boat like this?
It appears that the boat in the photos is in a marina, not your garage. Really, I suspect that size of your boat would make a difference.
If you are talking about a 17 footer, there is just not as much weight as the boat in the photo and you might get away with it. Note the phrase "Get away with it."
However, we have a tendency to underestimate our boats. Because we take them on and off the trailer so easily, we forget that they weigh (except for really small boats) as much or more than a car. AND, we would never work under a car supported with only a jack. Thus, when working under a boat (not on a trailer) safety would dictate using stands.