Empirical Knowledge.  For twenty-some years I have gone to BWCA or Quetico for one or two week-long trips every summer.  I never filter my water and I have never gotten sick.  If you think of the water as -having- giardia in it, not just that it -might- have giardia then, you should be really careful about any contact with contaminated water whether that contact is washing your body, washing dishes, swimming, water bottles, tubing, everything. Can a person swim in contaminated water and not get giardia as long as you don't swallow any water while swimming?