Exactly as you describe, it is 15 metres of flex with a "male" connector at one end and a "female" at the other. The big marina cartels (MDL, Yacht Havens, etc) like to charge lots for a berth and then extra for electricity.
If we have a plain cable (like this one) they charge £1 per day but if we have an in-line electricity meter it is charged per unit (at 5 times the electricity board's rate, of course!).