Have you tried any paint stripper for bottom paint? Sanding it with a DA sander and a minimum of a 60 gallon compressor is probably the most popular method. Removing bottom paint is hard NASTY work. Sometimes the best tools are a pen and your checkbook!
Heres an easy guide: Figure how much time and materials it would take you to sand the entire bottom, allow your self $10.00 an hour for your time(sounds low) Add it all up and see what the price is..Than get a quote for having it soda blasted..figure the difference and there you have the real cost to you If the difference is reasonable to you, get it done, it probablly won't take them all day and you don't get messy till you paint Just my $.02