Before I believe it outright, is that even possible? I seem to remember that before the Small World refurb they had a problem with boats hitting the bottom of the water and getting stuck. I never heard of a Small World boat "sinking", though. If they bottomed out there, but never sank, I'd think that Pirates would have a similar depth.
If it did happen, and nobody was hurt, then pictures would be cool.
When I was working on Casey Jr. in the late 60's, one of the Storybook canal boats sunk, and everyone got wet. Someone had forgotten to close the gate that allows them to take the boasts off-line, leaving a gap in the track. A boat ran right off the end of the track and punched a hole in the hull. I knew the girl who was the operator of the boat. She was devastated.
Those boats sometimes have a bad habit of taking on water throughout the day, especially in the last two rows. Perhaps too much water got in the boat + it was poorly balanced? And while I also don't think that it's possible to fully sink them it is definitely possible for them to bottom out on one end or the other.