Steps to reproduce

1. Summon a bunch of minecarts in a line.
2. Summon a boat on top of the minecarts.
3. Ride the boat on top of the minecarts.

Expected result

The boat moves slowly because it is not in water.

Observed result

The boat moves quickly.

When a boat (any kind including chest boats) is on top of minecarts, the minecarts can support the boat and make it move as fast as on water. It only works when the carts are not on rails; placing rails under the minecarts slow the boat as on land.