I was sitting in my boat and when I put my leather boots on, it showed my head backwards even though I was facing forward.

Details:
When in a boat, the horizontal angle of the player's head when viewed in the armor menu is related to the position of the boat in the water, not the position of the player's head relative to the rest of his/her body. The vertical angle of the player's head when viewed in the armor menu does not at all track the vertical angle of the player's head relative to the rest of his/her body.

I expected the head of the player in the armor menu to always face forward as it does when in a minecart, or to be the position of the head relative to the player's body (both vertical and horizontal)

How to reproduce this: Get in a boat, open the armor menu and see the player's horizontal head angle. Now paddle the boat a little bit on one side to turn the boat, being careful not to move the player's head. Check the armor view again and you'll see the player's head position in the armor menu has moved and is inconsistent with the player's head position in game.