Final answer:
The greater atmospheric pressure is under water due to the combined effects of atmospheric pressure and the pressure from the weight of the water above, with water's higher density leading to higher pressure compared to air at the same depth.
Step-by-step explanation:
The location with greater atmospheric pressure between a location 100 ft below sea level and under water is under water due to higher water density. Atmospheric pressure is the pressure due to the weight of the air above a given point and it decreases with altitude. However, the pressure exerted on a body under water not only includes the atmospheric pressure but also the pressure due to the weight of water above that point. Since water is approximately 775 times as dense as air, a small increase in depth underwater will result in a much greater increase in pressure compared to a change in altitude in the air.