The ships have tillers on the left. Since most people were right handed, the tiller was on the right, or StarBoard side. Board refers to the Tiller. The tiller prevented ships from docking on their Right side; hence the other side (left) was called PORT.
Ubbe and Floki are watching the sunset over the ocean. The sun does not set over the ocean on the east coast of North America, where the vikings have landed.