A calm sea surface gives little visual indication of the pressure farther below it. Depth changes the amount of water above a point, adding to the pressure experienced there.
Start with the surface contribution
NOAA’s pressure explanation describes an increase of roughly one atmosphere for each ten metres of seawater depth. Atmospheric pressure at the surface is already present, so added water pressure and total pressure are not identical quantities.
Label a classroom sketch carefully
Draw a vertical line with a surface mark and two depth marks. Label whether your numbers represent the increase from the surface or an approximate total. This avoids treating “one atmosphere added” as “one atmosphere altogether”.
The sketch is a conceptual aid, not a plan for entering the water or specifying equipment. Real diving and engineering decisions require suitable training, procedures and context. For general ocean reading, the useful lesson is simpler: a photograph of peaceful water cannot tell you the physical conditions that organisms and instruments encounter at depth.