My guess without pictures is its old barrier coat, which may be blistering itself.
Several approaches to dealing with the concern...
How are you stripping the bottom?
- Chemical Stripper, Scraper, Sanding, Sand/Soda Blasting?
You could just ignore it and paint and launch and worry about uit another day but since you're taking the time to strip the paint, if you have the time, might as well tackle this project.
Here is a portion of a thread where I posted how I dealt with our blister issue http://www.sailnet.com/forums/showth...=322678#322678
Pictures of the bottom would be helpful