Well it fits the description and the pits which are down to maybe 3mm at worst are really round and regular; that's what got me thinking., it looks like something's grown there and it's too regular to persuade me it's galvanic.
I can see the pits as I'm in dock at the mo but coming out tomorrow. There's about a foot of water in the bottom otherwise I'd grab some photos.
I did a module on biodegradation at uni in the dim and distant past so I'd heard of metal eating bugs although they were long forgotten until parasal mentioned them.
It's no biggie, I won't sink anytime soon, I need to come out again anyway as the "blacking in nearly zero temp" experiment was a disaster; most of the second coat is in the dock (but that's another thread. ), and I can prepare rather than having a nasty surprise... plus a Good pressure washing in an oxygen rich atmosphere will probably have killed it anyway. Even if it is galvanic it still needs fixing. And a coat of blacking will have stopped it in it's tracks for now.
Dan BA Hons (Silver Linings)
I've had an idea... drill them out, tap the holes, cap screws in and cap welded then when anyone asks pretend I gave up on the "rivets" plan when it became clear the welder was pissed