I'm not saying Bear is out in left field on this, because the rectal delivery of fluids (proctoclysis) is a valid technique that while "icky" (which I believe is the technical term!) can be quite the lifesaver if its all you have left to you. However from my research it is most useful for people who are sick/nauseated and can't drink, or for people who need fluid resuscitation but there are no IV supplies or medical expertise.
Case in point, check out
this story of a Nepali man suffering from shock in a remote mountain environment. The doctors had no IV fluids to administer, so they turned to proctoclysis and in their words "we believe it is unlikely that he would have survived without this intervention". It took their medical expertise, though, to know that the guy even needed resuscitation.
Of course there is the question of what will and won't pass through the intestinal tissue. From my reading, salt does pass this barrier (per the excellent book
"Essentials of Sea Survival") . So while a fluid that is mildly stronger than normal saline will probably be OK (i.e. the Robertson's slightly tainted rainwater), you can't just shoot seawater through there and expect to be fine.