The physical characteristics of this young man are all over the map from the initial news item to the current NAMUS. Since they've had ample time to verify his height and weight then we have to assume that the lower ranges must be correct.
I go over this bridge quite a lot, having a place in Gulfport. Several years ago a man stopped and threw his daughter over the bridge. A trooper jumped in after her but couldn't find her. This was on the southbound approach bridge called the Dick Misener Bridge which is well before you reached the higher spans. The water surface is about 60 feet from the bridge.
So if this young man did commit suicide it doesn't necessarily mean he did it from the highest span nearly 2 miles away. He could have walked into the water and just kept swimming out until he was too tired to return safely.
When the new bridge opened in the mid 1980's Florida decided to keep the approaches from the old bridge. Today they are considered the longest piers in the world used daily by fisherman. I believe they are state parks now, with bait shops, snack bar, and rest rooms open 24 hours a day.
Since the original bridge was still around when this young man was found it's possible he was fishing off the bridge and fell in. It's also possible that he was in a small fishing boat like a runabout, fishing near the pier and the boat got swamped by wake. There are so many variables as to what happened to him.