One GFCI is enough to protect an entire electric circuit. Building codes now require that you install GFCI outlets near water sources. The outlets do not have to be in the immediate vicinity of the water source, but they must be installed on the circuit that passes close to the water source. They should be installed as the first outlet in the series so the entire circuit is protected if any outlet should overload or short.
Before touching any outlets or wiring, disconnect the power to the circuit at the main breaker box. You can quickly verify that the main breaker box is wired correctly if all black (hot) wires are connected to the breakers and all white wires are connected to neutral.
Use a circuit tester on the outlet you plan to replace to verify that the power is off. Remove the receptacle plate and the old receptacle from its casing, then disconnect its wiring.
Connect the black wire from the incoming power line to the black connection on the GFCI marked "Line." Connect the white wire from the incoming power line to the white connection on the GFCI marked "Line." Connect the ground wire (bare copper wire) to the green screw on the receptacle.
If there is a second cable, connect its black wire to the black connection on the GFCI marked "Load" and its white wire to the white connection on the GFCI marked "Load."
Neatly fold back the wires and set the new GFCI outlet back into its receptacle box. Attach the receptacle plate.
Turn the power back on at the main breaker box and plug an appliance such as a hair dryer into the new GFCI outlet. Turn on the appliance and press the "Test" button on the GFCI; the power should shut off. Press the "Reset" button on the GFCI, and the power should return to the appliance.