The Titan does not have an externally opening hatch at all. The surface crew actually bolts and torques the front section of the submersible in place.

Once/if the Titan is located (itself a huge challenge), the only way the Titan can be rescued is by either repairing the submersible underwater so that it can float to the surface under its own power or by affixing a 4km tow cable to it and winching it to the surface. Only at the surface can they crack it open without killing everyone onboard.