The #minecraft:can_turn_in_boats entity type tag was added in 1.20.3-pre1, only being used in the experimental data pack for the breeze. Adding mobs like pigs or zombies to the tag works as expected, where they can turn freely when inside a boat. However when adding players, the player can still only turn halfway (-90 to 90 degrees) in the boat. I would expect the player to be able to make a full turn, just like other entities.

How to reproduce

1. Create a new world with a data pack
2. Add the file data/minecraft/tags/entity_types/can_turn_in_boats.json with the following contents:

   {
     "values": [
       "minecraft:player",
       "minecraft:pig",
       "minecraft:zombie"
     ]
   }
   

3. Reload the game /reload
4. Put a pig and a zombie in a boat
5. Notice that when holding a carrot the pig's body turns towards the player, and when in survival the zombie turns towards the player
6. Enter the boat as a player
7. Notice that you are not able to turn fully around