What is one to do if there is no water available?
One is required to search for water up to a Mil’s distance from his location [i.e. 18/24 minutes] to perform Mayim Achronim. If water is not available for this distance, then one is to wipe his hands on earth or a rock, or anything else that cleans. If he later reaches water after Birchas Hamazon, he is to wash his hands, as explained next.
If one forgot to perform Mayim Achronim before Birchas Hamazon, is it to be performed afterwards?
Yes. He is to perform Mayim Achronim after Birchas Hamazon.
 M”B 181:2; Kaf Hachaim 181:7
 Chesed Lealafim 181:8; Ben Ish Chaiy Shlach 11; Kaf Hachaim 181:26