Well, yes, they are counterfeit watches - massively less expensive than the genuine articles, so to think they are manufactured to the same standards is folly.
For scuba diving, many owners of gen watches would in fact wear a Seiko Monster or equivalent, or a backup dive computer. Rolexes are expensive things to scratch or bust while you're lugging around tanks.
The good news is that the PO's can be made very water resistant, but I wouldn't trust a collector to do this for you, because you want to see the test results with your own eyes. There are a few things that need to be done, like gluing down the He valve.
Besides, are you actually planning on diving deeper than 50m? Most recreational scuba divers don't go deeper than 30m.