As the unit of the depth is not mentioned, I will take it as 210 m.
The pressure exerted by a fluid of density ρ at depth h is given by,
P= P₀ + ρgh
here, ρ= density of water= 1000 kg/m³
g = acceleration due to gravity= 9.8 m/s
h= 210 m
P₀= atmospheric pressure at sea level = 101325 N/m²
Substituting the values,
P= 2159325 N/m²= 21.3 atm
Therefore, water pressure at the base of the hoover dam is 21.3 atm.