This is not being in a boat, but rather standing on top of one!!! The other day I was messing around with creating elevators using the boat hitbox and discovered an issue which means that pistons cannot push boats that have other entities stack (not riding) on top of them. Using the current bug report system it is no longer possible to mark older versions as affected but this has existed since the introduction of pistons (see screenshots (Beta 1.7)).

The bug can be easily reproduced by activating a piston with a boat on it and then retrying but with a player standing on top of the boat. I have tested other entities and only the player seems to affect the issue. If you want my opinion of the cause (I have no coding expertise), just say so. Bubble Columns have a similar effect and are very strange. If you need a video for the bubble columns specifically I can conjure one up.

The first image in each sequence is of the setup, the second of the boat without player standing on top, and third with the player on top (the boat does not travel up and just gets stuck in piston head) The last Attachment is a proof that this exists in the newest pre-release of 1.16 (pre 5).