Alright, so you know the temp change and you know the specific heat of water. Now to calculate the heat all you need is mass. Mass here refers to the mass of water since you're calculating the heat gained by the water. Note that because of the conservation of energy, the heat gained by the water must equal the heat lost by the man.
So, you are given the volume of water. Given that the density of water is about 1g/mL and that 1 gallon = 3.79 liters, you calculate the mass of water and use that mass to give you the heat gained by the water?