I reset the shore power breaker, and then waited for hours as the battery voltage slowly worked its way towards normal.
I have a Skymate system and it has a monitoring capability. My Simon monitoring system spotted that the voltage was low, but had no way to tell me. Skymate is a relatively low cost way of sending email that works about anywhere in the world. I have Skymate set to send me an email whenever the voltage is low or the boat loses shore power. As it turns out, Skymate did send me an email letting me know about the problem, but it was trapped as spam. I fixed that, and also programmed Skymate to trigger a phone call to me.
At dinner tonight, my phone rang, and it was a robot voice telling me that shorepower was disconnected on the boat. Argh! About 15 minutes later I received an email from the boat saying all was well. We were in the middle of Thanksgiving dinner, so I waited until after dinner and went to the boat. The power was on fine. My guess is that Skymate worked correctly, and the power was off for 15 minutes. All should be well now — but, the big question is: did I ruin my batteries?