Boats can enter minecarts and continue momentum when off rails

On both singleplayer vanilla and multiplayer spigot builds in 1.14, a boat can be placed in a minecart. If 1 entity is placed inside the boat the momentum will continue off the rails, and the direction of travel will remain constant. If you obstruct the path and push the cart/boat in another direction it will correct itself.

 

See video for setup and clarity in my description