A slow leak that filled the sub would not necessarily have lead to the vessel's destruction. That said, you can get leaks. I participated on a research cruise

A slow leak that filled the sub would not necessarily have lead to the vessel's destruction. That said, you can get leaks. I participated on a research cruise with a French submersible, and there was talk of small leaks with the main hatch seal occurring during descent. The "fix" was to go deeper so the pressure would essentially seal the hatch shut.