Hello there,
I was watching some interesting youtube videos of people building underwater drones like mini unmaned submarines,

which wonder : why can't we do the same transition we did from rc plans to quadcopters and just "fly" a reversed quadcopter underwater ?

It won't be able to rely on radio transmition, but ppm or sbus protocol over wire should work just fine, same for video a coax cable should do the trick,

If I use a hard tank as a flotter, it's internal pression hence its flottability should not vary with depth, so the drone will always tend to get back to the surface and I'll need to propel it downward same as we do in the air but reversed.

what do you think ? 

I'd be curious to give it a try

Sounds like a fun project.

Here is a video of a guy with a machine shop building an underwater stingray.
Looks a bit deep.

What sort of RC link do you use underwater? I don't think ELRS or crossfire will cut it.

According to a quick google search:
VLF (Very Low Frequency), or sound, or light (ie laser and optosensor), or you make a little floating pod with your rx and run a cable down to the sub.

Yeah, I assumed sonar, ELF or VLF or something along that line. But for hobbyists, probably way outside of available budgets and technologically beyond the level of probably the vast majority. Light would probably work as well, but wouldn't have the range.

Surface tether would probably be the easiest, but comes with a snag risk. But for a hobby level sub, probably a good choice (or just have an aerial poke out of the water, like a periscope).