You can attach a lead to a boat to pull it upward off the ground. However, if you try this with a boat that's in water, the lead will break instead. This makes it impossible to transport a mob via boat across a body of water and back onto land.

This is not a case of moving too fast and putting too much tension on the lead, causing it to break. Even if you try to do it while climbing stairs carefully, the lead breaks if the boat is in water.

The same thing happens in either Creative or Survival mode, and regardless of whether the boat is empty or is holding one or two mobs.

Steps to reproduce:

1. In Creative, place a boat on solid ground.
2. Attach a lead to the boat.
3. Fly upward in a series of short movements.
   Result: The boat leaves the ground and bounces and swings on the lead.
4. Place a boat on water and attach a lead to it.
5. fly upward as before.

Actual results:
The lead breaks at the point it cannot stretch any farther.

Expected results:
The boat leaves the water and bounces and swings on the lead.