How do boats get in the sea after being built?

Boats are hauled from the factory to the water on trucks and dropped into the water off a boat slip or using a crane.
Ships are built in either a cradle or a dry dock in the shipyard. The ship reaches a point where it will float and they slide it out of the cradle into the water, a very impressive site and one that is often shown occurring after the ship's christening. If in a dry dock it is floaded, floating the new vessel onto the water.