Steering wheels in modern boats are on the starboard side because that's where the danger zone is. That is, the arc from dead ahead to 22.5 degrees abaft the starboard beam. So that's the part of the helmsman's field of view that is most important to avoiding collisions. If another boat approaches yours in this arc, the Rules of the Road require you to give way. It's also why the starboard running light is green (for "GO") and the port running light is red (for "STOP").
Reference the International Rules of the Road, Rule 21, Definitions.
