It looks like that you have a weak place in the fiberglass under the gel-coat. You need to make a patch to the inside with a fiberglass repair kit. Then clean off all of the cracked gel-coat and fill the weak area with marine-tex. You will probably have to use a different fiberglass product, that won't sag, to build the area up to the proper contour. And then you could apply a new coat of gel-coat, or paint
I like to add shredded fiberglass cloth to the marine-tex to give it more strength in areas over 1/8 inch deep.
Be sure to get all of the cracked gel-coat off or it will just crack again.