My opinion: there is a difference between morality and social permissibility. Due to something called Dunbar's Number, a primate can only see about 150 people at a time as being real. Therefore, while the people in Darfur and the people on the submarine are both equally real, we only care about the kids on the submarine because they have sustenance in our minds. We know more about them, so they are more real. Not giving air to the kids is worse simply because air is more important than food or whatever it is our money gives to the poor folk in Darfur, but not giving money to charities is really just as bad, but culture in general permits it due to the fact that the people the charities help don't exist in our minds. It is socially permissible, but that doesn't make it okay.
On a lighter note: why the hell would you need six scuba tanks to go diving? Or even four? Two is too many! The amount of time he'll need to spend decompressing...
Also, a flaw I see in the question: if the dive shop cares enough to install a new button, why can't they just donate oxygen?