For creating water vehicles, you will need to first have an object you want to use as one.
Once you have the item in your scene, click on it either in the scene window or under the hierarchy.
Under the Inspector window, click on Add Component. Search in the search bar for Boat Vehicle.
This will add the boat vehicle component script to your object, as well as the Buoyancy, Boat Physics and Rigid Body components.
Add an animation for the vehicle under the Boat Vehicle component.
Set these accordingly and test it in the Game window in Unity with a playcharacter until you are happy with the outcome.