Its what you get with new shoes,
Or if it is on a grp hull, probably caused by osmosis, moisture absorbed into the laminate creating pressure forming a "blister"in the gelcoat
Is it not a navigation dome like the one you got on old WW2 bombers so that navigator could take a sight with out sticking head in air stream . similar for boat
From a well known 'experts' booklet on the subject
"One version is 'a small swelling filled with liquid".
On GRP hulls, they usually come into various categories:
(1) Gel Coat Void Blisters
(2) Interface Blisters
(3) Fibre Aligned Blisters
(4) Prominant Fibre Blisters ( Wicking)
(5) Inter Laminate Blisters
(6) Fresh Water Blisters
Usually spotted visually by an experienced surveyor, in spite of moisture meters (particularly on a hull just lifted out. Usual concern is 'how extensive? & how expensive?'