Plot hole: Whilst on the surface a drifting ship is seen and an officer and two sailors are sent to check it. On deck the officer says check fore and aft. The sailor who goes to the right and out of sight is back in 5 seconds. This is not enough time to check his allocated section. The script should have shown the officer check his watch (dial visible) and say something like "you have got 15 minutes". Next shot to show the officer checking his watch again and we see 10 minutes have passed and the men return.
Plot hole: The Seaview gets a mine cable snagged on its searchlight housing. Admiral Nelson says they are too deep for frogmen, use the mini-sub. As the two crew are entering they are seen to be wearing full scuba gear (less flippers).This suggests the mini-sub is not watertight and the above remarks by Admiral Nelson would apply to them also.