Firstly, use the PDF document provided by Microsoft to check a list with designated water runways in the Flight Simulator X. In order to move/stop your plane to one of these runways, click Free Flight from the Main menu and press the Change button from the Current Location. Click Clear Filters button from the Select Airport menu, type a designated water runway ID from the PDF document in the Search Airports field and select a starting position from the Filters. Click OK, change the current aircraft to a hydroplane, set the weather, current time and season and press Fly Now!. Check the official web page for further information.