I was thinking of setting it in Australia anyway.
How far out do they take the divers out? Would it take long to sent a rescue crew out?
In Qld, they take divers out to the outer Great Barrier Reef -- places like Michaelmas Cay and Agincourt Reef are popular. Note that commercial trips to the GBR are licenced and regulated. The whole GBR is a marine park and commercial operators are restricted to mooring at certain spots.
In Western Australia, the most popular reef diving area would be around Ningaloo. It's best known for the whale sharks.
There are numerous popular diving spots along the south coast. (No coral reefs, but plenty of rocky ones.)
Commercial dive operators will take out beginners for lessons, but they usually take a lot of boat staff as well as one or more instructors.
Once they realised that people were missing, they'd send out rescue immediately -- helicopters, planes and boats. Or, more accurately, once the boat staff told the police there was a problem. You could give them a bit more time by having the operators trying to cover their arses by not telling the police, but motoring back out themselves. If the divers have drifted, then it'd take more than one dive boat to relocate them.
One of the problems in trying to strand them on the GBR is that there are relatively few 'safe' (ie sandy cays or rocky continental islands) areas rising above the water. Coral will cut you to pieces. It will slice through neoprene, esp. thin suits. Your divers might only be wearing stinger suits, which are even thinner.
How long do you want to strand them? For a short time, the Whitsunday Group might be fine. The islands are close to the coast, but there's a lot of them. (If you're not sure where they are, look for Airlie Beach on Google Maps and go east and south-east). Another area, which is much less accessible to commercial tours, is the east coast of Cape York Peninsula. This is very remote. (Find Cooktown and head N along the coast.)
If you wanted to lose them forever, send them on a private tour to the islands of the Kimberley in NW Western Australia. Find Broome (or Derby) and head NE.
Watch out for saltwater crocodiles. Sharks. Box jellyfish. Stonefish. Cone shells. Fire coral. And so on.