When air is injected into water, the air has a certain volume it will occupy, which depends on the temperature, pressure, and amount of air. Surface tension makes the edge of the air bubble want to be as small as possible with the constraint that it still contains all of the air. In other words, the energy of the bubble due to surface tension is proportional to the surface area between the air and the water. It can be mathematically proven that a sphere is the lowest surface area shape for containing a given volume, so the lowest energy state of a bubble is a sphere. Thus air bubbles are most stable as spheres. If two small air bubbles meet, they will combine to form a larger bubble because larger spheres have a lower surface area for the amount of volume so they have a lower energy. Of course, the air bubbles are less dense than water so they will rise upward and usually pop at the surface of the water.