"If you are going to sand blas
"If you are going to sand blast the bottom, you run the risk of removing the gel coat as well if you aren't VERY carefull. If you do this you will need to barrier coat the bottom. There is a special "barrier coat paint" (not really a paint) you will need to use.
Last time I had to do this, I used a chemical remover "Peel a way". There is a marine version. There is also a non marine version sold at "big box hardware stores" that is the same stuff, only cheaper. There is a "non toxic" version that is suitable for fiberglass. Not cheap, but safe. A bit messy, however, you won't end up with fine flakes of bottom paint all over the place which can be an environmental (and legal) problem in some places.
Pressure wash ( mind where the old paint ends up) a good first step. Also a fast once over with a scraper to remove any big, loose chunks."