The most interesting thing I ever found was a mo-ped that had been thrown off a train bridge. The first thing I did was call the police to report the find. It really should have been the second as they were so keen to impound it that they would not let me take even minimal efforts to drain it, flush it and preserve it.
90 days late they called me and said I could have it as they were unable to determine or contact the owner. (?) I declined the offer and advised that they could haul the rusted remains to the dump.
The second most interesting find was a clean aand shiny vial of cocaine found under another bridge, where it appeared to have been recently thrown prior to a traffic stop. This I carefully inserted into a plastic baggie sans any new fingerprints and took it to the local Police Dept. The desk sargent looked at it, took it out of the bag, dried it off with a kleenex and said "Hmm looks like cocaine...not much we can do with it though as there aren't any fingerprints on it". And then he handed it back to me and went back to his paper work. My response was "so...I'm supposed to keep it?" At which point he finally got a clue. I did not bother to inform him that finderprints actually hold up well in fresh water and that there may well have been prints on it right up to the point where he wiped them off.
The more ordinary stuff consists of a few pairs of sunglasses per year (about 1/3rd of which are spendy), 4-5 anchors per year, a few fishing poles and a watch or two per year (about 1/3 of which still work - it's hard to kill a timex or casio).
I dive under docks regularly, but by invitation only from whover is responsible for the docks. I am the guy who usually gets called when a boat cover goes missing or someone drops their wedding ring, prescription glasses or wallet off the dock.
You do not want to block boat traffic and you really need to have someone on the surface to advise the boaters that you are there. You also need to be aware of the various cables used to secure the docks and be cautious of the potential for huge amounts of monofilament and other entanglement hazards.