Yes you can, but make sure area is cleaned of all paint /oil/ and dirt also use recommended aluminum wire for your wire welder and check your amp settings. It would be best to try some test welding on a piece scrap aluminum first so you can get the hang of it first. It would not be good to burn a big hole in your boat.
If your not sure of doing this you might want to take it to a welding shop near you most will be able to repair it for a minimal fee.