The same shot of a sensor dropping out of the Hydronaut and plunging into the sea floor was used three times.
When the first sensor is deployed, the escape hatch/sensor room is shown being remotely pressurized to equalize the pressure between exterior and interior so the hatch can be opened, it shows a compressed gas cylinder being crushed due to the pressure differential (improbable but not impossible). Several times during the movie the escape hatch is used to enter and exit the submersible. Yet at no time is is shown the chamber being pressurized, and in fact it shows the scientists immediately opening interior hatches without depressurization or decompression staging.
On the deck of the submarine after swimming in the ocean, Maggie removes her bikini top leaving her back bare. As she puts on the bathrobe you can see the lines of a bra across her back under the bathrobe.
At the very end, the underwater volcano that they were trapped right next to is nothing more than a small wisp of smoke in the distance on the surface during the rescue. It looks nothing like the major eruption as seen from the airliner window.
It makes no sense to travel the world underwater at a snail's pace when they could have traveled much faster by surface ship and dropped the sub at specific locations to plant the sensors. Or even multiple ships and multiple mini-subs. There was such a sense of urgency at the start they should have used the fastest approach to plant the sensors. In addition, how are the sensors powered? They are deep in the ocean where there is very little light (for solar power) and there is no battery technology to power them for any length of time. It does no good to plant sensors that only last for a short time.
Standish skin-dives to rescue a sub in trouble, to a depth of 150 feet. When he returns to the surface, he swims up rapidly. As a professional, expert diver, Standish should know he'd be risking a case of "the bends" when not decompressing on his way up. However, he made the dive and return on only the air in his lungs and there would have been little or no risk of decompression sickness (DCS). DCS is caused by nitrogen bubbles collecting and remaining in one's body. It depends on the depth of the dive and the amount of time at that depth - more time at greater depth allows more nitrogen to accumulate. Given that Standish was down for such a short period of time, limited by the air in his lungs, the amount of nitrogen absorbed by his body would have been insignificant and thus it would be unlikely he'd be affected by DCS.
Near the end, the pilot of the airplane says the volcano is 130 degrees west longitude (and change), yet Gary Merrill tells Lloyd Bridges that it is at 103 degrees.
Keenan Wynn's character, Stahl, who lives completely under the sea, has a darker tan than Lloyd Bridges character, who is a professional diver.
Dr. Standish free dives to 150 ft to rescue the mini-submersible after the collision with the whale disables it. When the SCUBA rescue team arrives, Standish takes 2-3 breaths from the divers regulator and immediately heads to the surface outpacing his exhaled air. If Standish had not taken the breaths from compressed air at depth, this would be no problem as the air would have returned to normal sea level volume, but since he did breathe compressed air at 150 ft. (appx 5 ATM) that air would have expanded to 5 times the volume at the surface. Failure to exhale while surfacing causes the diver to suffer an over pressure of the lungs/thorax resulting in a blowout (pneumothorax) and not the bends which is not a factor at this depth and time on bottom.