It’s hard to tell from photos just how bad the old paint is, or what type of paint it is, but the overall condition of your old paint doesn’t look too bad to me, unless you’re into racing. If you’re a racer, I’d strip all the old bottom paint. If not, I’d suggest you wet sand the bottom overall, but spend more time on the areas with the orange-peel surface, as shown in your first photo. I used a green colored sanding disk that I get at WM, Fawcett’s and other places. They cost about $4.00 each, but I only used two to heavily wet sand my C&C 35. I thought machine sanding was the faster way of removing old bottom paint, but found that wet sanding removes it as fast, but it's actually easier, because you don't have to hold the weight of the sanding machine up against the hull for hours, and you don't have to wear a respirator.
If you only want to smooth the surface of the old paint, I'd wet sand it. If you want to remove all the old paint, and it isn't too thick, I'd use chemical stripper. If you want to remove a very thick layer of old paint, I'd suggest that you hire it done. Your marina manager knows a guy who does it.
Since you don’t know the type of paint that’s on it now, I’d play the odds. The probability is that the old existing paint is some kind of a hard finish paint that can be covered over with a reasonably good ablative bottom paint. I’d wet sand the bottom, and put one or two coats of ablative paint on it, and see how it looks after a year. If it doesn’t peel, you’re good to go. If it peels, then all you have to do is remove what’s left of one or two coats of ablative paint, which should be easy to remove.
If you strip the bottom, don’t use one of the ordinary paint strippers that are sold in most hardware stores. It’s too aggressive, and might damage the boat. If the bottom has gelcoat, it will dissolve the gelcoat if left on too long, but there are strippers that are specifically designed for fiberglass. Interlux makes one, but, like all "yachting supplies," it's very expensive.
The paint stripper that I use works as well as any I've found, costs about $25. a gallon, or less, and is available nationwide. I use Klean Strip paint remover. They make different types, but you want the type that is specifically designed for stripping paint from fiberglass. I have used it to strip two boats with excellent results. I couldn’t find a paint dealer in the Chesapeake Bay area that stocks it, but special-ordered it from a paint dealer. (The company’s website might suggest a local dealer for their products.) There's no really easy way to strip old bottom paint, unless you hire someone else to do it for you, but a paint stripper is the quickest, easiest way I have found.
