Not too deep!
Physics Narrative for 11-14
Pressure and depth
As stated earlier, the pressure in a liquid, such as water, increases with depth. You're also probably aware that this is true of the atmosphere, where altitude sickness sets in at a few thousand metres above sea level, as the density of air is much reduced. So in both liquids and gases, that is fluids, the pressure increases with increasing depth of fluid.
This is an additional complication that is best avoided in an introductory study of pressure. In particular, the relationship between area, force and pressure is simpler to deal with if there is only one pressure throughout the whole fluid. Throughout this episode we shall not deal with these changes of pressure with depth, always choosing our examples carefully so that the depth of fluid is not too great.