The total (absolute) pressure experienced by the swimmer is the sum of the atmospheric pressure acting on the surface and the gauge pressure due to the water column above.

1. Formula: $P_{total} = P_{atm} + \rho g h$
2. Calculate Gauge Pressure ($P_{gauge}$): $P_{gauge} = \rho g h$ Substituting the given values: $\rho = 10^3 \text{ kg m}^{-3}$, $g = 10 \text{ m s}^{-2}$, $h = 20 \text{ m}$. $P_{gauge} = 10^3 \times 10 \times 20 = 2 \times 10^5 \text{ Pa}$
3. Convert to Atmospheres: Given $1 \text{ atm} = 10^5 \text{ Pa}$. $P_{gauge} = \frac{2 \times 10^5}{10^5} \text{ atm} = 2 \text{ atm}$
4. Calculate Total Pressure: $P_{total} = 1 \text{ atm} (\text{atmospheric}) + 2 \text{ atm} (\text{due to depth}) = 3 \text{ atm}$