First off, you need to understand what atmospheric pressure is. In a nut shell, it's the ambient pressure (force) that surrounds an object. At sea level, that pressure is approximately 14 pounds per square inch ( one atmosphere). In other words, there's a square inch column of air above the object that has a mass of 14 pounds and that column extends from sea level and up to the point where the earth's atmosphere ends. That entire column weighs in at 14 pounds. That's your starting point.

Water has mass too. More per square inch than air which is why it's ambient pressure changes even more rapidly than air as related to depth below sea level. At sea level, we know that the atmospheric pressure is 1 atm. At a depth of 30 feet, there is now an atmospheric pressure surrounding an object of 2 atm ( 28 psi). One atm accounts for the mass of the actual (gas) atmosphere and one atm is the mass of one square inch of water from a depth of 30 feet to the surface. At 60 feet, atmospheric pressure is 3 atm. At 90 feet it's 4atm. At 120 it's 5atm and so on.

There's a subtle difference between salt versus fresh water because of their different densities, but to keep it simple you can use the above to give an approximate atm at a given depth.

Water has mass too. More per square inch than air which is why it's ambient pressure changes even more rapidly than air as related to depth below sea level. At sea level, we know that the atmospheric pressure is 1 atm. At a depth of 30 feet, there is now an atmospheric pressure surrounding an object of 2 atm ( 28 psi). One atm accounts for the mass of the actual (gas) atmosphere and one atm is the mass of one square inch of water from a depth of 30 feet to the surface. At 60 feet, atmospheric pressure is 3 atm. At 90 feet it's 4atm. At 120 it's 5atm and so on.

There's a subtle difference between salt versus fresh water because of their different densities, but to keep it simple you can use the above to give an approximate atm at a given depth.