When Nelson tries to take the Flying Sub to the surface, he passes the derelict submarine resting on a ledge. The motion of the Flying Sub disturbs it and it starts moving towards Seaview on a collision course, directly for the Observation Nose on the bow. (In fact, Crane can see the derelict approaching through the Observation Nose's viewports.)
Nelson uses the Flying Sub to knock the derelict off course and prevent the collision. The scene cuts to a view of the derelict hitting the ocean floor to Seaview's starboard while coming from the direction of Seaview's stern, not the bow.
Nelson uses the Flying Sub to knock the derelict off course and prevent the collision. The scene cuts to a view of the derelict hitting the ocean floor to Seaview's starboard while coming from the direction of Seaview's stern, not the bow.
After the Seaview sinks to the ocean floor, Adm. Nelson takes off on the Flying Sub. Since the Flying Sub is made to be very light (so that it can fly in the air), it would be impossible for it to take the massive pressure that also threatens to crush the much stronger Seaview itself.
After the Seaview rescues the one diver working on a disabled submarine the Seaview is forced to run from some errant torpedoes and then sink 2,500 feet down into a ocean trench. However, when the Seaview finally reaches the bottom, the disabled sub is also there, with no explanation as to how it got there.