Just thought of a couple more,
Years ago snorkelling at Lancelin Island I thought I had found something interesting looking in only a metre or so of water right near the shore.
When I got it to the surface I realised by the shape and weight it was a live shell (bomb) of some kind.
I placed it carefully down on some seaweed on the beach and reported it to the police, the area was barricaded off and the defence force flew up a couple of bomb squad guys in a chopper who decided it was stable enough that the didn't detonate it but retrieved it to try to work out where and when it had came from.
Snorkelling again, this time in Parakeet Bay at Rotto, when I thought I could see an old boat propeller on the bottom, I dived down to retrieve it but it turned out to be a vertabrae from a whale.
I donated it to the museum on the island but it turns out they are worth quite a bit, I wonder if it is still there?
boneclones.com/product/humpback-whale-vertebra-and-intervertebral-discs-KO-255