I personally like this issue. So, you're being chased on a mountaintop, and so you put a boat down, hop in, and paddle off the edge of a cliff. You survive, and make an escape. XD
The following is based on a decompiled version of MC 1.12 using mcp940.
Please link this in the description of the report.
In the method net.minecraft.entity.item.EntityBoat.updateFallState() you have a piece of code that will always set the fall distance to 0 if the boat is not on land, its actually just wrong placed in the method.
This issue relates to MC-98160.