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Why is water pressure at a depth of 10 metres about twice atmospheric pressure?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

In fluid mechanics, the total pressure at any point in a liquid is the sum of the atmospheric pressure acting on the surface and the hydrostatic pressure exerted by the weight of the water column above that point.12

At the surface of a body of water, the pressure is equal to one atmosphere (approximately $10^{5}$ pascals). As you descend, the weight of the water adds to this initial pressure. Because water is significantly denser than air—with a density of approximately $10^{3}$ kilograms per cubic metre—the hydrostatic pressure increases by one atmosphere for every 10 metres of depth.123

Consequently, at a depth of 10 metres, the total absolute pressure is the sum of the surface atmospheric pressure (1 atmosphere) and the additional hydrostatic pressure from the 10-metre water column (1 atmosphere), resulting in a total of approximately twice the atmospheric pressure.13