Does not appear to happen on creative mode
Many times upon exiting a boat the boat will float off in a (seemingly) random direction, at varying speeds. Not sure if this is an intended game mechanic or a bug, but it really is a nuisance, especially when attempting to navigate a river and the boat decides to run into the shoreline and break.
I tested this in various ways: got in the boat and did not move my mouse (so I was facing the same direction as the boat) and dismounted, then got in and looked to the side and dismounted. It seemed to be random both ways. Each time I tested I waited til the x and z coordinates had stopped changing (so I knew the bug was not simply the boat moving in the direction it was going when I dismounted)
The only guess I would have for a cause is the bobbing moving the y coordinate may be affecting the effect of dismounting.
I would include pictures if they were relevant, but it's kind of difficult to get a picture of what's happening