A "tarred side" would refer to the side of a ship. Many times, ships were tarred to insure that they would not leak.

A "grey slap of water" simply refers to the water hitting against the side of the ship. The tar could force a sound that would sound like a slap given the tar forms a sort of skin over the wood.

Your reference to the Dark Continent could be true given interpretation is based upon a reader's understanding or justifications made.

The text in question comes from Graham Greene's novel A Journey Without Maps. As stated, the ship slips away from England: "England slipped away from the port-hole, a stone stage, a tarred side, a slap of grey water against the glass."

Here, one can surely see the movement of the ship away from the port of England given the imagery associated with England slipping away from the port-hole. (The port-hole is a window upon the ship and as the ship moves away from England it would look as if England was "slipping" away--getting smaller.)

The stone stage, though, is far more thought provoking. Historically, a stone stage was used during periods of time where plays were performed outside upon slabs of rock. These slabs of rock were either natural or placed. The stone stage was then used to perform upon.

Here, the stone stage refers to England. It is used as a synonym for the country. One could justify this based upon the hard and set ways of the country. When set in opposition against the unknowns of Africa, one could assume that the stone stage (the living in England) is very different from how living will be in Africa. Africa is unknown to the travellers upon the ship. Its history, for them, has not been set in stone.