I find it easier to do it in steps

Work out the volume in litres of the thing to move, that'll be the volume of water thats displaced. U

Using 1kg/l (or 1.03 kgl) the weight of the water displaced is approximately the same as the volume of the water displaced.

To be neutral the object displacing the water should eb the same weight.

To go up it should be lighter, to go down it should be heavier.

Thats the basic bit, so onwards to lifting something.

If you have something at the bottom you want to lift that ways 2kg and occupies 1kg of volume, according to the above it is 1kg negative.

Therefore to make it neutral you need to displace a further 1kg of water so you would need a container of about 1 liter (in fresh water etc) filled with air.

The amount of gas needed to fill the object would be determined by depth, so at the surface you need 1 surface equivalent litres of air, at ten meters you need 2 surface equivalent litres of air, and so on.

The reason for 'surface equivalence' is that at the respective depths they are always 1 litre.

So for an example, an object weighing 1000kg with a volume of 700L is sunk in fresh water at 30m.

What size lifting bag would be required to make it neutral?

What volume of gas would be required to fill the bag at that depth?

If you have a 12L cylinder with you for lifting the object, what pressure would it need to have to contain enough gas to do the lift?

Sorry if all that is useless but it was interesting for me