^{Update from EVGENSYPERPRO}
Summary:
The boat slows down in the air above incomplete blocks that do not contact with the boat (even without a hitbox, such as a flowers).
This can especially become a problem in the construction of "fast roads" using ice (when saving ice, players create a gap of 1 block that usually fill with slabs).

Steps to reproduce:

1. Build two stripes from ice blocks, between which the distance is 1 block.
2. Place incomplete blocks between them (for example, buttons or lower slabs).
3. Place the boat in the center of the building and try to drive over incomplete blocks.
4. Break incomplete blocks and try to drive again.

Observed results:
Incomplete blocks do not contact the boat, but they still slow down it.

Expected results:
Blocks not in contact with the boat will not slow down it.

Screenshots:

Original description:
Using a boat on an ice highway doesn't work as intended. The boat speeds up whenever it goes over ice, but as soon as it is over air, it slows down, and the cycle of speeding up and slowing down repeats over and over again.

You can see it here.