Best reason I've heard of is this, but I'll guess there's many more....
Before boats had rudders on their centerline, boats were steered by use of a specialized oar. This oar was held by a sailor located towards the stern (back) of the boat. However, like most of the rest of society, 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) used to stand on the right side of the boat.
Here in the UK its the same as cars so nooooo problem
