This model submarine, made with an empty washing up liquid bottle, shows how buoyancy tanks work.
Then tape 4 or 5 heavy coins to either end. This will add weight to the craft and keep the holes facing the bottom.
Attach the flexy straw using modelling clay if necessary.
To allow the sub to deep dive, add a length of plastic tubing to the end of the straw.
Once launched, the submarine will start to sink as water floods through the holes along the bottom.
Keep the end of the plastic tube above the waterline at all times.
Blow through the tube to fill the submarine with air. It will start to rise as the water gets blown out.
With practice you will be able to make the submarine float at any depth you wish.