So, what happened?

Corroded contacts. Pull the battery connections and clean them, thoroughly.
Check that the cables themselves aren't corroded. Then check the contacts
on the breaker and your main battery switch.

As was suggested in another post, it sounds like the repeated switching of
the breaker likely scraped through enough of the corrosion to make contact.
But if that's what happened then it's likely there's enough corrosion
elsewhere to merit addressing it NOW, before something else craps out on the
water again.