After one of the story missions there is a forced switch to Trevor, when he wakes up on an island off the southeast coast of Los Santos, in his underwear, with several dead bodies around him.
I was messing around with the boats nearby, dove into the water, and spotted an item lying among some rusted underwater structures. When I swam over the item, nothing happened except the sound effect that plays whenever you pick up something of interest. No collection counter appeared, no money was earned, no other visible changes occurred.
The approximate location of this mysterious item is indicated by the red dot:
The map shown here is from the progress tracker on Rockstar's Social Club site. According to that map, there is no trackable item located there. Since nuclear waste isn't tracked on this map, I thought it could be one of those. However, according to this other map, there is nuclear waste located nearby, but not where I found this item. Also I think you need to be in the sub to collect waste... I haven't explored that yet so I'm not sure.
What did I pick up?