In 15.8 (haven't been using water paths since that version so I don't know about 15.9 and 15.10) it was possible to hit a boat that had a horse on it. Hitting the boat to break it to an item was the only way I know how to get a horse off the boat. In 16.0 all hits that target the boat damage the riding horse instead, making it impossible to get the horse off the boat. I've tried to even hit it from underneath, which makes no difference.
1- Put a horse on a boat.
2- Ride the boat a bit
3- Get off the boat
4- Try to break the boat by hitting it.
Boat takes damage, and with enough damage it drops as an item.
Horse takes damage, boat is undamaged.