had the same problem, upon further investigation i realized that the prior owner had painted boat but did not use a primer. As much as I didn't want to do the whole sanding/stripping thing, I didn't want to bust my tail to do a good paint job to have it come off in a years time. I tried everything from hand sanding, using various grinder/sanding attachments w/ power drill & grinder, and everything else.
The fastest and easiest method to completely get paint off was something I stumbled upon here or another site. Go to advanced auto and get airplane paint remover. You're gonna need googles/glasses, gloves, and a face mask. (DO NOT SKIP this step, this stuff burns like crazy if it gets on your skin) Take the airplane paint remover and apply it all over with a big brush. Let it sit for about 15 minutes and you'll see the paint start to bubble. I'd wait about 20 minutes total and then get your pressure washer (or rent one) and blast all the paint off. It is by far the fastest way to get this done.
However, you may want to do this in an area that's not gonna drain into an area potentially causing pollution to water or animals. Also, I learned the hard way, this stuff will kill grass. After I blasted all the paint off I took the grinder w/ sanding bit on it and cleaned up the spots where needed. Then I laid down rustoleum primer, let it dry, and used an awesome base color I got from boater's world called duck boat green.