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:
Go in a boat in a body of water
place water one block above the water level and one back
Ride the boat into the flowing water one block above the water level
you will see your boat sink, then move
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ok
I can confirm, I have also seen this multiple times.
I can confirm for 20w46a.

This is the expected behaviour; see for example this comment by @unknown:
If you get water in the boat, you will sink.
Following the most recent comment, this is a duplicate of MC-89944 which was later resolved as "Works As Intended".
Please provide a screenshot of your game with the F3 debug screen enabled.