[uygi]

This may not be the best technical explanation of how it works, but afaik it's all you could ever need practically:

Boats don't zone and take you with them. They're mobs, you just happen to zone in sync with them. But when they reach the edge of the zone, they open a "portal" in which everything is zoned. Don't believe me? Go hang out where the boat zones without actually being on the boat. It'll zone you to the other side with the boat. Sounds like a local problem to me, either a client issue or a latency issue.