Yo,
Just browsing movie reviews and I can't believe you didn't understand Bo's deal with her water. Everything in those last 10 minutes is supposed to make you go "Ohhhh! THAT'S why!" and join Mel in his "religious realization." I would have loved this film if it stuck more with the alien thing and less with the religious crap. Also if the crappy "ummmm they just uhh.....left" ending. The deal with Merril and Morgan is explained (as far as his asthma not allowing the poison to enter his lungs and Merril's habit of "always swinging for the fences" allowing him to kick the crap outta the alien). Bo's is left for you to get on your own. Her deal with thinking her water is "contaminated" deals with its poisoning effect on these aliens (which is the DUMBEST thing I have ever heard but whatever!) and her habit of leaving them around the living room/house creates a "mine field" to the alien, which proves to be how he is "vanquished." So there you go...