When attempting to teleport a boat with a player in it using the command "/tp @e[type=Boat] ..." in a command block the boat will not teleport unless it is further than 10 blocks in the z direction from the point to which it is being teleported, holding that the x-coordinate is the same. As an example, if a player was riding a boat with a custom name of "A" starting at 0,0,0 and riding straight until 0,0,10 with a repeating command block which is set to "always active" and contains the following command: "tp @e[type=Boat,name=A] 0 0 0", the boat would not teleport until the player reached the position 0,0,10 at which time the boat would teleport and the player would continue riding it from 0,0,0. However, if the player were to travel in a curved path from 0,0,0 to 0,0,10 and change the value of its x-coordinate to 2, then the boat would teleport as soon as his x-coordinate changed.

Also, trying the exact same command in the chat will immediately teleport the boat although an impulse command block with the same command will not teleport the boat when activated unless it holds a different x-coordinate or is further than 10 blocks from the position to be teleported to, just like the repeating command block