when you go into flowing water that is above the boats current water y, it proceeds to sink and slow down, then kick you out here is link b/c the video was to big: https://imgur.com/a/hcqt6Pc

How to reproduce: 

1. Go in a boat in a body of water
2. place water one block above the water level and one back 
3. Ride the boat into the flowing water one block above the water level 
4. you will see your boat sink, then move