After using a boat to transport a horse and getting out of the boat it's impossible to re enter the boat... you always ride the horse! If you try to break the boat, you only hit the horse. You can only recover the boat after you kill your horse. But it works well with other mobs...
MC-92478
Horses are getting stuck in boats
Resolved
Duplicate
1
Rodrigo
(Unassigned)
Unconfirmed
Minecraft 15w45a
Retrieved
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-89963. You can find or add further information on that report.
If you haven't, please search before reporting any bugs, as it's likely that one exists already.