Well there's not much in the way of details. Unconscious people sink pretty quickly. Who knows what else was involved. If the fellow fell of while anchored, then I'd say it sounded odd. If he fell off the back of a fast moving boat and wasn't noticed missing for a few moments, then the story seems more plausible. If they whole bunch was somewhat tuned up, that wouldn't help.
There are alot of details that bridge the gap between reasonable and fishy. And firefighters have no special dispensation from drowning- it can and has happened to some of the strongest swimmers.
Another case where a life jacket would have probably made a dramatic difference.