If I understand you one way of doing this would be after starting up FSX and selecting your plane and the place you want to start at engines on or cold and dark your choice. After you have every thing set the way you want it then save the flight and make sure to check the box at the bottom to make it your default flight then when you start up FSX it will be with that flight 8-)
Gary M Buska SYSTEM Specs ASUS P8Z68 V/GEN 3 mother board: INTELL I7 2600k 3.48 ghz Quad core CPU with Sandy bridge: 12 Gigs of 1800hz ram: GTX 950 OVER CLOCKED: 2 Gigs Ram Windows 10 Home 64 bit Operating system. 750W Dedicated modular power supply. Two Internal 1TB hard drives 1 External 1TB 3.2 USB hard drive. SAITEK Cessna flight Yoke with throttles. CH Rudder Peddles 27 inch Wide screen Monitor
Select the Aircraft and choose a location - An airport close to a lake or sea. After loading open the Map and then drag the plane icon to the lake. Then start and fly! - I think this is OK.
Vista Ultimate 32bit Gigabyte GA-P35-DS3R Gigabyte GeForce 8600 GT 4 GB Memory 6400 Intel E6750 processor 500 GB HD SATA FSX Deluxe with Acceleration pack TRUSTMASTER Top Gun 2 Pro Joystick
a simple way (if it is on) use slew by hiting "Y" hit F4 to move up then F2 so stop then move your joystick around to move when you are over water hit F1 to go down the when you it the water hit "Y" agean.
In Alaska there are a lot of water runways. You can find them very easilly by looking at the map, instead of the traditionnal symbol, they have an anchor. When you select one of them, you will notice that in the choice of runways, there is written "water" next to the runway number
You can make a simple test by choosing the easiest one: Seattle Seaplanes