Well, one possibility is to consider purchasing a refurbished camera. The makers above sell cheap underwater cameras directly to the public from their refurbishment sites.  Now keep in mind that they don't publicize this a lot because they want to sell you a new camera (higher margin).  Some of them even sell their refurbished cameras through third parties. The fact is you can find them with a few searches on the internet.

You can also scour the classified adds to find deals.  Underwater digital cameras are not overly common items on the classified ads so you will have to set up tools to the searching for you.  Some of the classified ad sites have functionality to set up keyword searches and notifications when something is found.  You just have to set up an account and put in yourkey words and wait for a notification that someone has listed a cheap underwater camera for sale.

Just keep in mind that purchasing directly from another individual carries the most risk.  Rarely will they warrant the performance of the item. It is "buyer beware".  For this reason it is prudent to insist on the best (cheapest) price on these types of transactions.  That way you are not out too much money if the camera doesn't work and the seller disappears.

Another great source of cheap underwater cameras is EBay.  The structured EBay auctions give you a much better chance of a fair exchange. EBay has some great deals but they also have some junk that is sold there.  The best approach to EBay is to come armed with questions and background information so that you can query the seller.  If you do this with some depth you will understand completely what you are purchasing.

These cameras are out there but they will take some homework on the part ofthe buyer to ensure getting a good deal.