Yup, what q said. Mine is a different boat, but it did the same thing when I got it. I used a dowel to poke down in the water, found a small hole that was plugged. so small I cant get my little finger to fit. I will be drilling it bigger this spring, and sealing the edges with some epoxy, now that it's dry.
Edit: Looking at the pic again, It looks like you should replace the fuel filler hose also. Looks like its starting to peel off the outer rubber.