As said by Mckenzie, if a person wants to go underwater, only a few grains of Aquaman crystals are needed because it already contain enough oxygen for one breath and as the material can absorb oxygen from the water, the diver will not need to bring more crystals.
One important thing about the Aquaman Crystal is that it does not react irreversibly with oxygen. The material itself is both a sensor and a container of oxygen. It can absorb and release oxygen a lot of times without losing its ability.
The Aquaman Crystal uses an element called cobalt, bound in an organic molecule. It gives the material the molecular and electronic structure that enables it to absorb oxygen from its surroundings.
But divers should be very careful not to breathe the material below 15 feet although it is still safe but never breathe it below 30 feet because it gets toxic and it can even kill you.