A quick skim of the facts of the lake make it easier to understand, its a huge salt water lake that receives fresh water from various rivers and empties I believe into the gulf of mexico, it referenced shipping lanes and tidal changes as well which would make more sense as to how this could happen. He could have been injured from the accident as well and strugglin to get to shore would really wear down the body as well.
Lake Pontchartrain - Wikipedia, the free encyclopedia