You need at least a quart of water, more is better. Whatever your choice, water bottles should not leak if you turn them upside down and leave them that way for a while. I use pills, Potable Aqua. My friends have filters. Either work pretty well, and both methods have limitations to their effectiveness.
Pills are lighter but add a flavor to the water.
Filters are heavier, but don't impart any taste to the water. Never assume water in the field is potable regardless of what the sign says. Boiling works, but it is a nuisance. There are problems with filters being difficult to pump, and clogging in the field. The reviewers gave high marks to the PUR Hiker, and it still got positive comments in the March 1998 edition of that magazine.