JoshS,
With regard to your original question, "Is [a salt water enema] a legit technique for staying hydrated, or just a rumour?"
Although I'm be extremely wary of anything read on the internet, why allow these well informed rationalists to influence and spoil your enjoyment of the countryside? I'm sure there are a number of Bushcrafters who actively research such techniques... in fact I've even heard rumours of groups who dress up in cammo-gear and do just that.
The therapeutic value of colonic irrigation is well publicised and has become very popular over the past 20 or so years, and if it's good enough for certain television celebrities and the Royal Family, then it is most certainly good enough for Bushcrafters.
Having said that though, I'd be extremely wary of practicing the technique on my local beach or woodland due to the real possibility of being discovered holding a flotsam 2 ltr soft drinks bottle full of sea water, attached to a length of garden hose plugged into my rubber-ring-gear, as I think that one might just make the front page of the local newspaper!
Cheers,
Pango.