The easiest, least expensive way I've found for a do-it-yourselfer to remove old bottom paint is to use a paint stripper made specifically for use on fiberglass. I get it at a paint supply store that specializes in automobile paints. Don't use ordinary paint strippers, because they're too aggressive, and will damage the gel coat.
Spread a tarp under the boat, and brush the stripper on with a 4" brush. Don't scrape it off. Keep brushing additional coats of stripper on top of the old paint and prior coats of stripper. The objective is to keep it wet. After 3-4 coats of stripper, try to scrape the paint off a small area. When most of the paint comes off an area with fairly easy scraping, then it's ready to be scraped off. If the paint doesn't scrape off fairly easily, then apply more coats of stripper. When you scrape it, just let it drop on the tarp and when you're done, roll up the tarp and throw it away.
After the paint has been remoived, I wipe the area with acetone and then wash it with soapy water, rinsing it thoroughly.