Cool or tepid. Certainly not hot. However, be aware that if you haven't already developed blisters, they could show up after the shower. Be sure to gently pat dry, don't rub your skin with the towel. For one thing, if you do have blisters, your skin will heal better if the blisters are left intact and the fluid remains against the damaged skin. For another, it just hurts to rub burned skin.
Hot showers should not be taken if anyone has sunburn. The best is a cold shower since it helps in closing up the pores of the skin, which in turn traps in the heat. While a cool shower feels best, it may make the sunburn worse by holding the heat. A hot shower will not close the pores and the heat can escape. For more information, visit http://www.wisegeek.com/why-does-my-sunburn-look-worse-after-taking-a-shower.htm.