Since it's a Salt Water boat it could also be multiple layers of that yucky Ablative Bottom paint too!!! Make sure and wear a GOOD respirator when you're sanding on whatever it is!!!!
I agree with WOG on this before spending any money on repair supplies get it off the trailer and flipped over and start grinding onher get down to the original gelcoat or glass whichever shows up first. There is definitely some funky chit going on there. At least if you get ir all ground down and figure out its not worth repairing all you have wasted is time a little money on the grinding discs.Based on what I'm hearing and seeing, if I wanted to restore this boat, my plan of attack would be to strip everything off of her, get her off the trailer, upside down onto a cradle and grind all the crud off that hull so I could see everything that needed to be repaired. Fix all of it, Paint or gelcoat the hull then flip her right side up and do the rest of the restoration. That's how I would go about it.
But hey, I'm just an...so you might want to consider the source.