Very similar experience in the BVIs about four years ago. Two of the 8 batteries boiled out.
Luckily we were docked and I got them ashore quickly but after 6 hours they were still hot and bubbly, even completely disconnected.
The fault is usually internal with the battery and not necessarily a charging issue.
We ran just fine on the remaining six (I believe you could live on 4 conservatively) until I could find a compatible replacement pair (very expensive min the BVIs) and then, a couple of.months later replaced the other 6.
Since I couldn't find exact matches for the first two replacements it seemed would that they always took way more water to replenish and ran a little warmer than the other 6. I know you're not supposed to mix / mis-match but it's what we could do at the time.
Replaced all eight less.than two years later, surely because of the mis-match but also because I knew that this was temporary.
Now have 8 Trojan 30xhs and couldn't be happier.
I also installed a two probe Taylor temperature alarm in hopes of getting a heads-up if anything like that happens again.
I'm pretty fastidious about checking and adding distilled water on the first day of every month now :)
SM 209, Adagio
Block island, USA