The underside of my Exocet is coming up in dozens of tiny little bubbles/blisters 1-2mm across. Is it serious? What can I do about it?
They had that sort of trouble years ago with fibrelass swimming pools. Lots of small blisters. In fact some ended up about an inch across. (2.54cm for the metricated)
I think it turned out that there were microscopic holes in the resin top coat and water was being forced in by osmosis, the same process that trees use to suck up water.
Real good for trees but not so good for swimming pools, surfboards or windsurfers. 
Sounds like it might be something similar. The only way to stop it is to never leave your board in a wet bag. Otherwise you would need to sand off the original topcoat and re do it. That's what they did with swimming pools because obviously you can't leave a swimming pool dry.
I always leave mine in a wet bag because I'm such a caring and careful person with all my gear, not,( Borat not joke) and so far, no problems, so I think there's something wrong with your board.
I bought an ex-hire board with a nose protector on it and when I removed the protector, there were tiny bubbles under the paint from water trapped under the nose protector. The water can soak through the paint, given enough time.
I decided that if the water can get in, it can get out and just kept it dry. It seemed to reduce a bit over time, but didn't disappear. It was only a small area and very small bubbles, so I didn't sand it down, dry it properly and repaint it.
Throw away the nose protector and repair the nose.... noseprotector is a marketing thingie and it causes more harm to your board than having a board without it....
So, I should keep it dry and think about block sanding it back and painting it over winter?
