I've made a really realistic torpedo. It goes underwater (you can change the depth in the designer), its engines work underwater, it has realistic speed about 40 mph.

How to use it:

1) detach cockpit and save as template
2) 2 yellow cones above the torpedo mark the bouancy element that controlls the depth. to attach a torpedo under a plane - move the boyancy element with FineTuner further up and delete the cones after you find a boyancy element
3) attach to a plane/boat
4) Move the boyancy element with the FineTuner at the desired height above the torpedo body (its nocollision) - it will go right on the watersurface with a torpedo hanging below
5) if you want to change steering input - go to rear vertical fin and either change or delet the steering (if you want an uncontrolled ww1-ww2 torpedo)

INGAME:

1) switch to a camera on the torpedo
2) Detach a torpedo (detacher currently at AG1)
3) move throttle to at least 1% to engage torpedo engines (it will go top spead at any throttle setting apart from 0)
4) control the torpedo with YAW (you can change the steering input at the rear vertical fin)

Can sink any ship 1 hit (including USS Beast)

p.s. unfortunately, if you put throttle at 0, when a torpedo is underway, it will reverse for some time before it stops