I don't think seadog was referring to the well known shipwrecks of boats that actually ran aground in close proximity to island as per the link above.
I lived on the Island for a number of years and had some involvement with the Fremantle Historic Boat Museum so have read a lot of the stories of how most of them ended up where they now are. There is some interesting stories such as the story of the City of York. I have dived on most of them and you can actually get a good look at most of the ones listed by just snorkelling on a good day.
However there is an area approximately 10k's SW of the Island and anywhere between 50-80 metres of water where lots of ships/barges etc were scuttled and it is referred to as the Rottnest ships graveyard, I think this is the area you were referring to wasn't it seadog? I had a mate that dived on several of the wrecks and did some photography for National Geographic (deep dives using special gas mixes) and he gave me several co-ordinates that we punched into my GPS, found the wrecks and fished them. I have since sold my boat but did record all the co-ordinates ......somewhere? I will see if I can find them and PM/email them to you.
Some are mentioned in this link. http://www.museum.wa.gov.au/collections/maritime/march/documents/No.%20148%20Graveyard.pdf
