Just like serious birders and all manner of naturalists, many divers keep a life list of the species they have seen. Typically, the more elusive the species, the more coveted the sighting. Size also plays a role in desirability, whether it’s a condor or a blue whale.
When it comes to impressively large and elusive freshwater species, a couple come to mind.
Great Lakes Now
https://www.greatlakesnow.org/2022/01/meeting-mysterious-muskie/