The Woods Hole Oceanographic Institution uses a US Navy submersible, the DSV-2 Alvin, which first made a descent to the bottom in 1965, and has made over 5,000 dives since then, including to the Titanic wreck. Of course every element of its construction has been replaced over the years. The US Navy has other submersibles that they don't talk about. Of course none of these submersibles were subjected to regulations when they were built, just a spare no expense attitude. James Cameron has safely visited the Titanic wreck 33 times in his personal submersible, and he has that attitude.