Answer
Raise the temperature of $4.0 \times 10^{8}$ kgs of water by 50 Celsius degrees.
Work Step by Step
Calculate the energy released by the explosion.
$$\frac{(20 kilotons)(4.2 \times 10^{12} J/kiloton)}{4,184 J/kilocalorie} = 2.0 \times 10^{10} kilocalories$$
This is enough energy to raise the temperature of $2.0 \times 10^{10}$ kg of water by one Celsius degree. Equivalently, this energy could raise the temperature of 1/50 of that amount of water, $4.0 \times 10^{8}$ kgs, by 50 Celsius degrees. As stated in the question, this is about half a million tons.
