The bug

Boats inside non full level water (see MC-124819) sink even if NoGravity is set to 1b.

How to reproduce

1. Create a straight waterfall
2. Summon a boat with NoGravity:1b

   /summon boat ~ ~ ~ {NoGravity:1b}
   

3. Push or steer the boat into the water
   It floats or sinks

Code analysis

Based on 1.12.2 decompiled using MCP 9.40

It looks like the reason could be that when a boat has the status EntityBoat.Status.UNDER_FLOWING_WATER, the method net.minecraft.entity.item.EntityBoat.updateMotion() modifies d1 which is added to the y-motion instead of modifying d2.