This bugs been around for a while now, but Smallbirds and Lavae, both followers, can't board boats!
Obviously for Lavae you wouldn't want them on your boat for a long time but they should still be allowed to cross over them for easy maneuvering.
This is especially bad cause if you hatch them on the boat they'll be stuck forever!
Hatch the love of your life, a smallbird/Lavae!
Try to spend a nice evening on a boat with them, realize they can't board! Panic!
Okay but actually, hatch a follower using their respective egg and realize they get stuck infinitely trying to board boats you're on.
Other mobs like Cats and Frogs can board them, they should be able to, too.