So this sounds like it was just a big paint bubble. I would plan on stripping the paint off the rudder and apply new paint properly before launching in the spring.Again, thank you all for your comments. I went out to the yard this weekend to discover that the blisters have disappeared (which I gather translates to the blisters were in the paint* rather than the laminate). I will go out again in the next couple of weeks** to drill hole in the bottom of the rudder and measure amount of water that comes out. Is there a proper technique for drilling a hole in the rudder? I'm planning to use 1/4" drill at the lowest part of rudder going in from bottom (rather than side). I don't know the construction of the rudders but presumably I won't hit a water pipe/electrical line (or equivalent) doing that. (?)
I am one that sees no problem with drilling a hole in the bottom of the rudder to see how much water is inside. I've done it. Just need to inject epoxy (use the West Products syringes) and you seal the hole. And no, you won't hit any water or electrical lines.