If somebody said
きれいな みずを ください
what is being asked for?
a. mineral water in a bottle
b. tap water
c. water straight from a beautiful spring
d. water straight from beautiful sea
If I say it in Japan or Australia, I mean b. tap water. “a. mineral water in a bottle” is of course accepted but it doesn’t have to be that special.
きれいな is often remembered as “pretty” but here it means “pure” and “clean,” so if it is made sure that it doesn’t contain any harmful chemical and/or germs, that is acceptable. Water straight from a beautiful lake does not guarantee it’s free of those and sea water contains salt and other things.
Other usages of きれい include:
きれいな人（ひと） = a (visually) beautiful person
きれいな字（じ）= tidy hand-writing (not necessarily beautiful)
きれいなへや = a tidy room, a clean room
きれいな選挙 = a clean election (without any corruption)
Don’t forget that きれい is a なadjective even though it ends in い.
