I've created a report on this issue before, but in this report, I found more detail about the issue. A lead on a mob would at any moment break at a chunk boarder. When this happens, the boat rowing sound stops as well the horse swimming sound until the horse unfreezes. This must be why leads sometimes would break when pulling a horse with a boat. These two bug reports MC-145404,
and MC-141564
are related to this issue.
Please provide a small video with the debug screen enabled (hit the
F3
key) demonstrating the issue.