Choose a boat and simply slew it over to water

When you start on a runway enter the map. You can do this either by pressing the appropriate button (it is usually on the cockpit next to the ATC communication one) or press ALT key and from the bar on the top of the screen go to WORLD-->MAP (someone could correct me if I am wrong as I do not have the FS at hand right now). In the map you see your position as well as the position of the tower. Zoom in so that you can distinguish your position from the tower's position, click on the little airplane (that's you) and drag to the sea. The little airplane is now above the sea. Do not forget to set the altitude on the left of the map to zero in order to avoid a falling down. Press the OK button and that's it, you are floating.