A two inch blister is huge. I would get a glass guy to look and determine what kind they are just like Faster said. For the most part blisters are benign, just a negotiating point. With the exception of new or almost new boats, all boats have or have had blisters. The good news is you can't see them when you are sailing!
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217