I've built this a while ago at the start of the challenge, but didn't want to upload it for fear of it being plagiarized. Anyways:

This thing is very versatile and can go anywhere with ease. It has few controls and they are all useful (except maybe AG3)

VTOL: Control the direction of the jet engines. Point up for a vertical take-off or landing and activate AG1. Recommended: Point up the engines when landing in both VTOL or plane mode to control descent easily.
TRIM UP: Plane Mode (use both mid-air at 250mph or on the ground)
G: Boat Mode (Make sure to activate AG1 to steer with Yaw)
AG1: Horizontal stabilizer - use for control in boat mode, when flying (if you need to straighten out quickly), when landing (helps a lot) or when driving at high speeds in a level place.
AG2: Boost (Useful for quickly building up speed, especially when taking off and transforming into plane mode mid-air - not recommended for long use because it's overkill)
AG3: Emergency Parachute because why not?
Also has air-brakes
WARNING: Take off vertically when taking off in water, (if you want to deploy plane mode in mid-air, do it at a speed of 250mph and a sufficient altitude).
Also don't forget to turn off AG1 and G when going to land from water
This thing is armed to the teeth, so you should have no problem dealing with any type of enemy.
BONUS: Infinite fuel and cockpit
When you get the hang of this thing, you'll be doing all sorts of crazy stunts in no time.