How do I get an animal out of a boat?

The only way to get the cow out of the boat is by destroying the boat.

If the cow entered there accidentally,remove the boat by hitting it(don't hit the cow!),and don't let it get back in the boat again,if you do not want this trouble again.

You can't hit the boat on the sides or else it hits the animal.

I was transporting my horse and had to get under the boat AND hit the boat in the right spot (seemed to be a bottom corner).

Amazingly I just used a lead to remove a dog from a boat. I haven't thoroughly tested this, but you happen to have a lead, use it on the animal (you may or may not need to 'lead it out').

You need to use a lead to make the cow/animal to stand up, but don't walk away because then it will just get back in the boat. When on the lead, the animal will just kind of stand on top of the boat, then you can destroy the boat with a sword without hurting the animal.

Adding to Bob's answer, you can use F3+B to view the hitboxes of the boat and the animal, to make sure when breaking the boat you don't accidentally hurt the animal too