By assuming the fluid is incompressible and at rest, the hydrostatic pressure distribution is expressed as:

     p₁ = ρgh + p₀

From standard handbooks, it is noted that the atmospheric pressure at the sea level is p₀ = 101.3 kPa and the acceleration of gravity is g = 9.8 m/s². At fresh water temperature of 16^oC, the density of water = 999 kg/m³ and the density of seawater = 1030 kg/m³.

The absolute pressure at a depth of 12 meters of water is
     p₁ = (999 kg/m³) (9.8 m/s²) (12 m) + 101.3 kPa
         = 219 kPa

whereas for seawater:
     p₁ = (1030 kg/m³) (9.8 m/s²) (12 m) + 101.3 kPa
         = 222 kPa

Therefore, it can be concluded that the density of the fluid (fresh water vs. seawater) in this scenario plays a very small role (1%) in the design criteria.