When placing a boat into water you sometimes cannot place a boat.
To replicate go into 1 block deep water and try to place a boat at various angles and distances from your player. Observe that boats sometimes don't place. This affects both creative and survival mode.
Video of the bug: https://www.youtube.com/watch?v=yBhjrJbuEXw&feature=youtu.be
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Can confirm (maybe when the player's hitbox is in the way, similar to blocks). Also I remember watching your "Things you (possibly) didn't know about Minecraft" series on Youtube 🙂.
Since entities can now push the player and the new boats placement being more consistent than the old implementation (as of 15w43a) I think that the inability to place boats inside the player's hitbox is intended. If a mod could mark this as WAI it would be much appreciated.
This may be related to MC-47555 which was labeled as fixed in 14w07a. However, if that is the case this is WAI even though it means we must jump to place boats in water at certain distances and angles.