Originally Posted by DIXIEDOG
The reason is because most propellers are right hand rotation and if the driver is alone having their weight on the right side of the boat helps keep the boat more level on take off with a right hand prop.
Partially true but there is an older answer....
Quoted from Captkujo.com...
You might be thinking that it has to do with most people being right-handed, or with using the driver’s weight to counteract prop torque. Both are true. Partially.
For the most part, all boat traffic keeps to the right, according to the International Regulations for Preventing Collisions at Sea. Before the use of the modern day rudder, boats were steered by a specialized oar, which was situated on the right-hand side, or starboard side of the vessel (originally steerboard). This oar was held by a sailor located towards the stern. As there were many more right-handed sailors than left-handed sailors, this meant that the right-handed sailors holding the steering oar (which had been broadened to provide better control) stood on the right side. The helmsman, standing in the middle of the boat and looking ahead, used his right hand to operate it. Traditionally, boats would also moor with the left hand side to the quay to prevent damage to the steering oar, and this was referred to as larboard (loading side), later replaced by port to prevent confusion from the similar sounding words. By keeping to the right, boats pass "port-to-port", protecting the steering oar. When modern style rudders fixed to the stern were developed, the helmsman was moved amidships (on the centreline), and when steering wheels replaced tillers this generally remained the same. Many motor yachts and other small craft are right hand drive, but some boats, typically smaller pleasure craft and wooden speedboats are built left hand drive to give a better view of approaching and passing traffic.