One factor in shipping may be that captains are only at the helm for limited time (albeit important moments), and typically have a helmsman or two to count on. The captain would take the helm going into and out of port. His helmsman would be steering at most other times, and seeing as ships pass port-side to port-side, it would make sense that the helmsman is on the left.
What A laugh. I was 5 years Merchant Marine and never ever, and never will see a master at the helm. He and the Pilot are normaly on the wing of the bridge checking progress. Steering a ship is easy, giving commands to a helmsman is not, and is left to the Pilot or Master