Can a person be under a sea for 30mins
A dead person. I think you would drown. But some people can stay underwater for like 5 minutes because they are trained and they keep practicing and eventually their lungs adjust to being underwater for long times. :D
Yes, if they have diving gear, are in a tunnel or are in a submarine.
Without Scuba gear? No. With Scuba gear? Yes.There is always a chance of the Oxygen running out though, so you would have to hurry.
not yet, the world record is close to 20 mins