When cutting through the wall, the position of the cutting torch changes position several times.
Naval terminology calls mechanical doors, hatches, not doors. A door would lead to a cabin.
When watching the giant manta from the viewing windows, you can see the mantas fins breaking the surface of the water. This is when the Seaview was submerged.
While on the "bottom" the manta is shown striking the Seaview and also breaking the surface of the water.
At 26:05, in the upper right of the tank, you can see a gloved hand dropping pieces of dry ice into the tank with the baby manta.