They could have made it under these circumstances
I've recently watched the Mythbusters documentary on this escape attempt, plus a channel 5 documentary which also involved a slightly different re-enactment of the escape. Having watched the results of both the tests, I now believe it is possible for them to have reached the mainland, but only under certain circumstances. If they had been using a raft like the one seen in the film, they probably wouldn't have made it, especially if they were heading for Angel Island. The channel 5 documentary tried building a similar raft, however the 3 people on it didn't get very far from Angel Island, before the raft punctured, and they had to be rescued. Also the documentary said the current was running out of the Bay at the time of the escape, making it very difficult to reach the island on a raft. If they realised they couldn't get to Angel and instead to tried to ride the current to the Marin Headland, they might have made it, but if they were on the partially submarged raft (like the film), they'd probably be suffering from hyperthermia so much that they wouldn't be able to move if they did reach the shore. This is what happened to escapee John Paul Scott, who swam to the mainland, but was unable to move as his hyperthermia was so bad. If they did reach the shore in this condition, they would probably have been caught as they'd been unable to move.
Conversely, if they'd been using a raft like the one in the Mythbusters documentary, and hadn't been heading for Angel Island, and immediately gone to the Marin Headland by the Golden Gate, they could have made it. The Mythbusters team tried this using a raft where all the occupants were out of the water (unlike the film), and made it to the Marin Headland. However its also possible they were swept out to sea and drowned. I'd say their chances of making it would depend on what kind of raft they used, how well made it was, and where they were headed. Feel free to discuss.