There's some merit to this. The hatch not opening from the inside means that even if they did manage to reach the surface of the ocean, if they didn't find their way back to either their own mothership or something else capable of opening the hatch from the outside they still would have died due to lack of oxygen when the oxygen ran out.

I think the concern there is that they would be able to surface but not control the craft then drift somewhere and eventually suffocate.

It apparently had dissolving straps on a ballast so it would (almost) surface on its own in case of issues.

If it wasn’t an explosive event they would have been stuck just under the surface with no way to get out. That said, opening a hatch in that scenario would be an absolute last resort anyways. It’s not like the people that went overboard on the Titanic lasted long and it’s in the same exact place.