Is there any substantial and worthwhile difference? It seems to me that all water items should be put together.
All water purification items fall under water, but not all water items fall under water purification:
- How much water should I take per day on an average hike?
- How can I stop water from freezing?
- What do I need to look for in a good water bottle?
For that reason I think they should be kept separate.
I'd like to see water remain distinct for a bit unless we can establish how we might want to tag water-sports or water-activities such as canoeing, lake boating, lake sailing, ocean boating and sailing.
Water purification (or more broadly - establishing a secure, sanitary, reliable water source) is one of the big three survival criteria that each of us faces when we leave civilization and head outdoors.
I suppose a good tag wiki will help shape things once the site has moved past beta, but it seems a little early to equate water to water-purification to me (who has only recently arrived so may be unaware that this is already worked out somewhere but not yet in the tag wiki)