Could be poor or poorly prepped base/barrier coat, or could also be failing. I had that on my old boat, and yep, I think a bottom job could be in your future (ugh). That being the case you COULD be dealing with blisters in places, but those rings are too perfect to be anything but a bad/poorly faired repair (be it from filled in through-hulls or prior blister repair).
My personal opinion would be to have someone soda-blast it, and then from there they or you, fix blisters, re-fair, then barrier coat, and bottom coat. When it's done you'll be happy you did, but boy what a PITA if you don't have someone else blast the bottom for you.
Faster beat me too it.... but yeah thinner layers are better.