It's not a few seconds, it's a very small fraction of a second. The Thresher imploded in 1/20th of a second at 730 meters. We don't know for sure how far down Titan was when it imploded, but based on the time they lost signal, I'm guessing around 3500 meters, so we're talking about 4-5 times as much force. Plus the hull was made of extremely brittle carbon fiber, so it wouldn't buckle at all, it would just collapse all at once. It's hard to overstate how much force we're talking about; at that depth, it's about equivalent to building the Empire State building out of lead and sitting it on top of the ship with no other supports.

It's not just that they didn't have time to drown; it would have imploded so quickly that they would have been dead before their brains even had time to process that something was happening.