When a boat is placed on top of a slime block, the player is able to enter the boat and, the closer to the bottom of the boat that they enter, the farther up they get launched. The player will start rising horizontally, and can be launched at a maximum of about 100k blocks if they position themselves properly (with commands). Proof of this on youtube at https://youtu.be/yaQEsW1yBzs (could not fit into 10 MB attachment space)