1.15.2, Java 1.8.0_51 64bit, Windows 10
When a boat is riding another entity, the player can get in the boat and rotate the boat like other rideable entities that are riding another entity, and the boat's blue F3 + B rotation line appears to update with the boat's new rotation. However, the NBT Rotation tag doesn't update, instead only showing [0.0,0.0] (south) for rotation always, and other commands also treat boats as if they were facing south. I expected commands and NBT to show the correct rotation of the boat and the correct position/rotation of the entity riding it.
How to Reproduce - NBT
Summon a regular boat:
Jump in and spin around a bit. Observe that the Rotation NBT tag changes accordingly;
Summon a boat riding another entity:
Jump in the boat and spin around. Observe that the NBT tag for Rotation always shows [0.0f,0.0f], regardless of the actual rotation of the boat:
Using F3 + B, you can also see that the blue line, which represents the boat's rotation, updates accordingly while the NBT tag does not.
How to Reproduce - Passenger Position
To demonstrate that this isn't just an NBT issue, we will summon an armor stand with a boat riding it with another armor stand in that boat.
Note that the armor stand riding the boat has the tag "particle". Putting this in a repeating command block that is always active will show where commands think the armor stand is through a cloud particle:
See the cloud particle above the upper armor stand:
Jump in the boat. Notice that the particle's position changes accordingly when the armor stand is pushed back:
Now rotate the boat. Notice that the particle's position now fails to update according to the boat's rotation:
Break the boat's bottom armor stand. Now spin around and notice that the cloud particle's position now updates correctly:
How to Reproduce - Rotation Carets
This will put a cloud particle in the direction that every boat is looking:
Notice that for boats that are riding other entities, this will always face south, even if the boat is facing another direction.