If someone were going diving and you agree to pump oxygen for them but didn't and they end up drowning, would you be legally responsible?

First, when you say legally responsible, eo you mean criminally or civilly?  The standards are different, but really the answer to either is it depends on the circumstances.  Did you fall asleep after the person dove, or did you let the person know two weeks befroe the dive that you would lnot be able to pump oxygen?  Were you unable to pump oxygen because you had a heaert attack, or did you just decide you were mad at the diver?  If you were on the boat and just decided not to pump oxygen, knowing that the diver was relying on you to do so and that he/she would be in danger of drowning if you didn't, or perhaps even intending that he/she drown, you could be liable both criminally and civilly.  If you let the diver know a week in advance, so that he/she could have decided not to dive, you could be liable civilly for breaching your contract, but not criminally (althoug it is not clear what damages you'd be responsible for, perhaps the extra cost of hiring osmeone else to pump the oxygen; under those circumstances it's very unlikely that you'd be liable for wrongful death, although I wouldn't put it past some lawyer to sue you for it).  There are many other possible scenariox with differeing facts which could lead to different results.