Boatpox!

Yup. Boats can get sick. The above is a sick case of boat pox! :O Gasp!
No, actually it is called hull blisters and these need to be treated immediately.

So pull out the boat onto dry land, remove any sea creatures off the bottom of the boat, inspect for blisters, if any found pop them with a knife allowing the brownish ooze to seep out (it smells like vinegar...), take an electric sander and remove any wet paint and saturated fiberglass, and then let the hull dry... might take a loooong time. After it's dry, apply an epoxy resin to the damaged area, let that cure, then apply a fiberglass filler compound mixed with resin to fill in the holes, and let that cure. The sand the area smooth to match the texture of the boat and apply another coat of clear resin.

That's it. All it is to it... except kinda vague so if I were you I'd look the same instructions up elsewhere but in more detail. Sorry. That's how I condensed it.