After several hours of testing with torpedo designs, I want to present you my best result.
It comes with full launching equipment and is ready to run.
You only have to put it onto your creation. (you should remove the cockpit on it, it's just for uploading)

Cruise speed in water is 180 MPH, in the air it's ~ 600-700 MPH.

With 120 gallons of internal fuel it can run several minutes and together with it's 100% stability it allows for attacks from very huge distances.

The closer you are to the target, the lower you should launch it, because it has a very stable flight and will lose altitude very slowly.
I wouldn't recommend launching from above 1000 feet above sea-level, as the torpedo might get lost in the water.

Use AG1 to launch and AG4 to get rid of the remaining detacher after launch.

Feedback is appreciated!
Enjoy! :)