Get near a river, and setup a trap.
Means, you need to put signposts to trap an area, and let only one tile free, for the animal to be able to come in (hint : using ctrl + g in ender's client help with this).
Then pull your animal. get aggro'ed (from far with your sling), then run in this trap.
It will follow you, and get in too.
Now get into your boat, and go near the tile without signpost, but keep being unreachable by the animal you are trapping.
Then wait for the animal to "twitch" (i call that "minimove") every few second any animal check if it can reach you or not, doing so, it does a little movement.
Just after that minimove, go and signpost the entrance of the trap, and go back into your boat.
Here you go, your animal is trapped so it won't be able to flee
