Help

Try using wings as drone blades to create down and up draft motion.
Its a fake way to imitate buoyancy.

@Toinkove Thanks for the help

To be honest I dunno how viable submarines are for this game since to make a real submergible vehicle you need ballast tanks (an ability to allow in varying amounts of sea water into the craft to stay neutrally buoyant at a certain depth). but I would suggest using a wing placed near the crafts center of mass, but configure them more like horizontal stabilizers on a plane. so as you are under propulsion you can essentially force large amounts of water up to keep the sub submerged.

@Zeadwaffen I recreated it here:
https://www.simplerockets.com/c/k1SZKq/D-S-X-V1

I lost the build, sorry

It is always best to post the craft so other can examine it (like showing the mechanic your car rather then phoning them). However, are you making the canard from a fin? found out recently that does not work when making an aircraft rudder (based on a report). Try a wing with a control surface placed vertically or a structure panel.

Perhaps post the submarine. You give little information, too many conclusions can be drawn.