First of all, Fresh water is more likely to cause blisters than saltwater. (The molecules are smaller and react more readily to form acids) So if you didn''t have blisters in fresh water that tells you a little (which is positive) about the quality of the glass work.
Let me guess, you prescheduled trip was not only into salt water but warmer water than the fresh water where the boat had previously been kept (OK, So I have just been watching a Charlie Chan movie)
Putting a barrier coat on is not a bad idea. If you decide to put one on the boat needs to be very dry. In other words, you need to strip the bottom down to the gelcoat (or beyond) and let it sit for months in a dry location. It is a big job to do right.
Bead or walnut shell blasting is a better technique than sanding because the heat from a major sanding job can accellerate or initiate a blister problem even in a boat that is out of the water but which has a reasonably high moisture content.
If I were in your shoes I would probably just repair the couple blisters that you have and monitor them at next haul out to see if the blister problems have progressed. If the problem abates, your home free. If it gets a little worse then make the necesary repairs. But if it gets much worse, then do a peel, and come back with epoxy and cloth which is the real way to do it right.