When I say popping open, what I mean is the whole blister will break off leaving a small circle of exposed fiber, not just release the air/fluid inside and settle down. Osmosis also takes longer to develop normally. You can tell if it is by bursting one of the blisters. If moisture comes out and it smells like vinager then it is osmosis. If the gel coat is very thin, then it could be a combination of the damp lockers and air expanding pushing the gelcoat out through pinholes in the laminate. To dry the lockers you will have to flush them. Salt water will never dry out fully as the salt remains and then reabsorbs moisture.

To me that looks like blisters in paint... The layer just seems way too thin to be gelcoat..  I would definitely wash out any traces of salt in the "lockers" and try to dry them out, maybe using some sort of tubing and a "boot dryer"
(The ones you can buy for drying your boots, with a small fan and heating element that blows warm air out through "tubes" you put your boots on..  Am i making any sense here... )

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Thanks dbcrx and Dravis, I think first things first then, getting it washed out and dried to hopefully stop it getting any worse, then wait for the warmer weather before attempting a repair. Thanks for the boot drier tip.

More than happy to do either, happy to help you repair it or do it all for you.

Thanks Tom, PM sent