The keel is that fin-like projection on the bottom of a ship. The intended was taken to the bow (that is, the front) of a ship under way (that is, moving) and tied to two lines (that is, ropes). The victim was then forced into the water. One line was walked down the port (that is, left) side of the ship and the other taken down the starboard (right). Keep in mind that old ships usually had barnacles on their hulls below the water line.

By all accounts, it is a very, very unpleasant way to to put to death.