Boats, boat with chest, bamboo raft or bamboo raft with chest can't be placed on top of fences, walls, and closed fence gates (when holding shift). With fence gates, it does place successfully when the fence is open.

This probably happens due to the fence's collision hitbox being 1.5 blocks tall while the hitbox used for targeting the block is a single block tall, meaning that the boat colides with the upper part of the fence's hitbox. This can be seen in-game when placing a boat on an open fence gate, as the boat appears on top of the gate rather than half a block above it.