In the snapshot 16w03a and previous, the hitbox of the horse was larger than the hitbox of the boat (the boat sink more and the horse's legs is out for under the boat), making it impossible to break the boat if a horse is riding passenger (bug report MC-89963). In 16w04a, it was solved thereby... If a mob's hitbox is too big it won't go into boats, the horses can not be mounted on the boat (bug report MC-96212) but, in the snapshot 16w04a / 16w05a, if you get put a horse in a boat (for example, with commands), now you can interact with the boat... The boat hitbox is below the horse hitbox (you hit the boat down and the horse will disembark because the boat does not sink and the horse's legs are inside the boat)... So, it is not necessary that the horses can not be mounted on the boat.

Test:
/summon Boat ~ ~ ~ {Passengers:[

]}
and, in creative or survival, you hit at the indicated place of the images or, directly under the boat (that is the area that can be hit).

I insist on this subject because the long explorations with transport (transport without infrastructure; minecart need infrastructure: rails, bridges, tunnels, etc.) in v1.9 would be ruined.
For example:
I ride my horse, I find the sea, I let my horse, I mount on a boat alone, I arrive another continent but without a horse...
Or:
I'll mining far from home, I fill the chest of my donkey, my house is on a remote island... I can not take my donkey.

For all these reasons... I think this bug report is important, I hope this bug report not will be labeled as "Works As Intended" and I think that the solution is not that horses can not be mounted on the boats; horses and boats are not problematic.

I do not speak English well, I hope the report is understood.

Thanks to everyone and special thanks to [Mod]redstonehelper and [Mojang]ProfMobius (present in other associated bug report).