Without knowing what it is coming from I can only throw some guesses out. I'm sure others may have other opinions or more information. Is it something that would show up in a picture? If so you should post one for us. Also is it sticky on the entire hull? Or is it localized to a small area?
In my mind I see two potential causes. One is a poor quality paint that was used that is now breaking down after exposure to sun or having been washed. The other; something else that was applied, who knows what. Either way I think that we all can agree that a nice smooth hull is best for efficiency and ability to keep clean.
Will you be re-painting the hull? If so whatever is on there is going to be an obstacle for adhesion and will need to be remedied before the new paint goes on or you are likely wasting your time. If it were me I'd start trying a few different things in a small test area, starting with the least aggressive options and working my way up. Obviously you can try warm water and soap. I am sure someone out there will say its a bad idea but you can try dawn dish soap, it is actually a very good degreaser. You can try a pressure washer, but I assume you already did that. If you have a heat gun you can try to warm it up a little to see if it softens. You can also try 99% isopropyl and scrub a little with something soft. Be aware that if the Isopropyl works you will need to change out your scrubbing medium regularly as it gets loaded up with whatever you are trying to remove and just starts to smear it around. If you go with isopropyl I recommend you buy that by the gallon online. There is a huge markup on that at the local drug stores. Although you can pick up a small bottle there for your test but the highest percent in the drug stores around here is usually 91% which is not ideal but it would still give you an idea as to whether it will help. 70% isopropyl is weak and might not do much for you. I believe acetone will harm the fiberglass so I'd hold short of that option. Keep in mind that as you move towards more aggressive cleaning agents and abrasives you will potentially be damaging the paint underneath, so if you use something aggressive you may end up needing to paint after whether or not you had originally planned to do so.
I'm sure that someone else can chime in and let you know if this is a good idea or not, but if you already plan to repaint you can see if it comes off with light sand paper. A lot of guys do a simple "scuff and shoot" paint job. Meaning you scuff the original paint with sandpaper and shoot it with fresh paint, skipping primer. When it comes to paint adhesion there are two types. Chemical and physical, this is why on the back of a lot of rattle cans the directions say re-coat within 30 minutes or allow to dry for "x" amount of hours then sand lightly before an additional coat. The scratches in the undercoat provide a rough surface for the top coat to adhere to. Recoating after 30 minutes is a chemical adhesion while the paint cells are still "open". When they dry the only option is physical adhesion which is why they recommend to abrade the surface before recoat at that point. This is why it is common to abrade the surface of older existing paint before applying new paint over top, which is why I say that if you plan to repaint anyhow you might see if it can be sanded off. That said if you can find another way to get the sticky stuff off first I would do that as it will load up your sandpaper and you will find yourself needing A LOT of sandpaper. I'm sure others will have a different opinion on this and I'm curious what they would have to say about it.
Please remember that when you use flammable chemicals to handle the rags appropriately. Soak them in a bucket of water before disposal. I have had spontaneous combustion happen from chemical-soaked rags in the past and it isn't fun. And don't forget to protect yourself. Wear safety glasses and gloves!
I'm dealing with a similar beast right now. Previous owner of my boat smeared kitchen/ bathroom caulk all over every seam and rivet. In my case soft scrub bleach is breaking it down well if I let it sit for 20 minutes before I start scraping with a stainless steel razor blade. (stainless steel is softer than common hardware store razor blades but you have to order them online). Not sure I would use a razor blade on a fiberglass boat though, mine is aluminum, I don't have as much experience with fiberglass boats to know whether you can scrape them with a razor or not.
I hope this helps! Let us know how it goes!
