After three seasons our 309 strangely had a huge blister on the keel. It had only been in fresh water and the season only involved 6 months each year in the water. The boat was properly cleaned and painted with VC 17 each year. It was stored indoors on a factory cradle for the winter season. The blister was about 8 in. in diameter. No one had a clue as to what caused it. For $95 the marina did an excellent job of grinding the blister from the lead keel, fill, fair, and paint (75.00 labor and 20.00 material)... but the marina already had the boat on the cradle and ready to work on.
When I had my boat hauled after its first full season in the water, the yard (which was also the selling Catalina dealer) noticed an area on the starboard bottom, a couple of feet square, that had groups of very tiny "pimples" in the gelcoat under the bottom paint. The dealer took it upon themselves to contact Catalina, who suspected there might have been a problem with the batch of gelcoat used on my boat. The following spring, they sanded off the bottom paint and gelcoat on the entire bottom, down to the laminate, and found no evidence of blisters or water intrusion or absorption into the laminate. They applied six coats of epoxy barrier coat, followed by two coats of ablative bottom paint. All was covered under warranty, and the only downside was a month delay in spring launching. That was done in 2008, and the boat has been in the water for 4 years (with one winter on the hard) without further problems. In fact, I was hauled 2 weeks ago for the hurricane, and did a close bottom inspection and all was well.
My 2007 had many tiny (about 1/8" across) blisters on the rudder after the first season. Dealer fixed under warranty by sanding and then applying a few coats of barrier coat. No problems since then. Which is amazing, considering the dealer is an imbecile.