the difference is that a bo'd person is alive, a drowned person is dead. big difference.
near drowning:
http://www.emedicine.com/ped/topic2570.htm
there is also the possibility of secondary drowning where a person had water entering the lungs during immersion but was regaining consciousness later on. this water, if not clinicly removed, will over time increeasingly inhibit gas exchange as the water moves around inside the lungs. eventually the person dies from not enough o2. if there is any possibility of water in the lungs, even if the diver doesn't feel it see immediate medical attention. secondary drowning can occur mayn hours after the event.
http://en.wikipedia.org/wiki/Drowning