Interesting.

I have a difficult time explaining your
bubbles in terms of galvanic currents,
since they occur mainly where stuff has
been welded on the inside of the hull.

My first reaction is to think in terms
of increased thermal mass in the welded
areas causing temperature differences.

Next thought is stresses on the hull at
those spots causing "stretching" of the
paint surface, maybe allowing water in.

Sorry I can't do better than that.

Good luck.

Chuck