Sand the old paint off down to the gel coat using 100 Grit wet/dry sandpaper.....Wet. Wear dishwashing type rubber gloves or you will loose the skin off your fingers. After that you
may find "spider web like" little cracks all over, which retain the old paint in the cracks and doesn't want to go away.
The cracks are NOT a problem.....BUT they look ugly. Paint the bottom again to make it look good.
What paint to use is another topic....depending on what you find when you sand. NB
PS: I recommend you look for another boat. This boat is WAY OVERDUE for replacement.
NB