Temperature is one of the most difficult things to measure. For example it is not just to put a thermometer inside an oven an do readings if you want a correct value. But if you are satisfied with an approximate value, this is ok.

If you release the bubbles in the deepest parts of ocean, below 8000 m, the bubbles would have so high density that they will sink instead of rise.

This because the air compressed to more than the pressure at that depth will have a density higher than water.

But probably will the bubbles dissolve rather quick, but a baloon will definitely sink. You could fill the Marianer trench with air! Maybe.