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.