With aluminum boats, blisters under the paint almost always mean that there are spots of corrosion developing. The corrosion produces an aluminum oxide 'powder' or residue that expands and pushed up the paint, causing the blister. If not treated, these corrosion spots CAN get pretty bad, even to the point of a hole in the hull.
The cure is about what you would think - a complete strip of the paint including sanding, coating the entire hull with a good epoxy primer, filling in any corrosion pits with an epoxy filler compound, putting down many coats of a really good barrier paint, and then your normal anti-fouling over the top of that.
It's a lot of labor, but a known process.
