floating boat question?

Upon playing various games, I wanted to know how they got that boat to float on a collisioned platform. is it placement of the boat upon spawned? All I want is some example of how it is done.

(Update) here's what im looking for Basicly when the boat spawns on the "water platform" how it moves around on it. The "ship" is generic and has no scripts in it except the script for moving around.