for what it's worth,
338 has those pocs, very simular, where ever I have sanded thru thr gelcoat - or almost thru the gelcoat - I'm only assuming that it is the nature of the gelcoat just after they sprayed it on and were laying on the mat - the first layer of the laminate. I've come across this topside and on deck.

When 338 was coming out of the water and being surveyed, Capt Roger found some five or six suspicious bumps in the antifouling. I forgot about them and they "diaappeared" The hull was taken down to the gelcoat some time later and they have remained disappeared. Have since found out that this is common. If the bumps were true blisters, they are still ther and will reappear.

The isophthalic polyester in our hulls does not produce the modern blisters. But obviously air pockets and/or incomplete mixing problems, foreign material or almost anything could cause isolated or uncommon problems in the A/C gelcoat.

The mat under the gelcoat on 338 doesn't seem to be very thick. Blisters are a phenomenon that appear at the interface of gelcoat and mat. An assumption I have to make is that any free chemicals that didn't get catalysed in the original hull layup MUST be long since gone.
Afterall the hull is semi=permeable, water flushed you could say.

Most of the blisters I've observed on other boats are symetrical like boils. They ARE boils, under pressure with chemical and salt water cocktail. What I see in your foto looks irregular, angular. Almost as if they are externally caused damage. Like an anchor? Halloween prank? Maybe badly patched by the DFO and reappearing. I would get close with a magnifying glass. Get some oblique light on the hull to see if ther are anymore bumps ready to hatch. But by definition, age. material it can't be your normal everyday blisters.

Personally, I would do two stage epoxy. Laminate Part A, Slow hardner Part B. Soak the holes, wipe them 'dry.' Fill with epoxy/cabosil gel. I disagree with polyester on polyester cold patching and underwater repairs.