In the US, I noticed that boats’ wheels are on the left. Why is that? Also, related to this, when passing through a channel or shipping lane, do boats keep right (in the US)? - Jinx 
They’re not always. I see as many RHD boats as LHD. My dad’s boat had RHD. (Mine’s an outboard with tiller steering.)
The nautical rules of the road (which are close to universal) specify that when two vessels meet head-on or nearly so and there might be risk of collision, each alters course to the right. (Not quite the same thing as saying that they keep right in a channel.)
Just theorizing?If a boat has a single, high-torque propeller rotating counterclockwise as seen from aft, it will tend to lift up the port side. Putting a body over there will help keep the boat level. That’s the same reason helicopters are flown from the right seat, while fixed-wing aircraft are flown from the left. A twin-screw boat wouldn’t have that problem, much, and a boater might feel more comfortable on the same side as if he were in a car.
Not sure which sorts of boats you’re talking about, but in the US the vast majority of pleasure craft have the wheel on the right.
Perhpas when we get up into the serious 75’ luxury yacht category the wheel moves to the centerline, but in general they’re on the right.
eg: http://bayliner.com/index.asp, http://www.searay.com/index.asp, etc. These guys http://www.hatterasyachts.com/about.cfm make moderate luxo stuff that moves the wheel to the centerline in the larger models.
I’m pretty sure the OP got his left and right mixed up. That wouldn’t have happened if he used the proper terminology, port and starboard for left and right.
I’m in the boat and yacht repair business, and I can’t recall ever seeing a helm on the port side. It is predominantly on the starboard or much less often on a centre console.
Hard to say why, but a lot of boats mount their transmission and throttle controls on the side of the boat adjacent to the helm. These controls can be quite stiff and it helps the majority right handed skippers to use their strong hand in order to manipulate them. I’m left handed and I always feel a little awkward on the controls.
I’m not in the boat business, but I do see LHD quite frequently. It surprised me at first, since when I was a child dad’s boat was RHD and I don’t specifically remember anyone else’s boat.
Dad didn’t bother with a transmission. He just had the screw connected to a blueprinted Olds V8. It was a heavy mahogany 20-footer, but it surprised a lot of people. 
I’m surprised too. He must have had rudders, but I’d be scared to take out a 20’ boat with out a reverse drive.