One atmosphere is also the pressure caused by the weight of a column of fresh water of approximately 10.3 m. A diver 25 m underwater experiences a pressure of about 3.5 atmospheres (1 atm of air plus 2.5 atm of water). The added pressure on tom's chest from the water would make inhaling low pressure air from the surface extremely difficult.
The answer would be 3 5/12, because you would make 3 2/3 into 3 8/12 for the greatest common factor and make 1/4 into 3/12 when you subtract 3/12 - 3 8/12 you get 3 5/12.