When we traveled to Costa Rica, my ex- and I brought back a textbook case of giardia. I was pretty much okay, my only symptoms were lack of appetite basically (like eating 2 bites and feeling totally full) but he got the whole thing (diarrhea vomiting etc.). I had taken some water from a supposedly "safe" source of water during a hike from which we both drank, but I think it was mostly "safe" for the locals who have the right stuff in their stomach to deal with the little buggers. I am usually pretty resistant to this sort of things, too, it would have taken me a few more weeks probably to realize I was sick if he hadn't had more serious symptoms.

Guatemala is a bit less advanced, public-infrastructure wise, than Costa Rica, so I'd be even more careful, drink only bottled water, take plenty of water on treks, etc. It'll be the dry season so be very careful to take PLENTY of water. The filter bottles are nice but they're a burden to carry around and, indeed, not as safe. As for iodine...yuck, I will only drink water with this stuff in case of advanced dehydration!!

I also recommend hep-A-B vaccine (one shot now, another in six months, usually) and bring some malaria medicine, too. Inquire at your local travellers' clinic about the presence of resistant sources of malaria, in which case a special type of medication is needed.

Enjoy the trip! Lucky you...