The bug

When placing a boat on blocks whose outline boxes are taller than their collision boxes (such as soul sand, snow layers, and honey blocks), the boat is spawned on the top of the outline box and falls down to the collision box. This is unlike spawn eggs, which spawn mobs directly on the collision box.

To reproduce

• Place a block that has different outline and collision heights (such as soul sand, snow layers or honey blocks)
• Place a boat on top of the block
  The boat appears to sink into the block.