I use muriatic acid on things that are old and have been allowed to corrode, and when the only other option would be taking a wire brush on the grinder to them.
The fittings need to be taken off the boat and the acid is pretty strong, so it needs to be handled carefully, but it works quickly and it's possible to end up with a smooth finish, unlike that obtained through brushing. I never use it on aluminum - it turns it black.
With old brass and stainless things that are in really bad condition, it saves a lot of time. Have never taken it on or near the boat, and would not advise it. But if you are working on a really old boat and need to clean something that hasn't been touched for ten or fifteen years, it works quickly. You will still need to polish it, as it comes out of the acid bath looking matte and stained.