What I expected to happen was...:
The boat disappears and you are teleported to the right place.
What actually happened was...:
You become invisible, you fall through the ground. When it reaches the height -66.05000 you stop and cease to interact with the world.
Steps to Reproduce
Put the boat abroad of the world.
Sit down in it.
Go to the edge of the world, sitting on the boat.
Environment
Windows 8.1
Fixed in 16w15b, now you return to 29999999 coordinate when reaching 30000010 and don't fall through the textures. The boat sometimes is destroyed in the sticks and wooden planks. The player or mobs no longer become invisible when the boundaries of the world.