Obvious statement alert: water is a key element on Earth. Water is also a key element in Annie John. There's water water everywhere, and yeah there are drops to drink… but the symbolically important stuff is bathwater so eew: don't drink it.
One of the first key bonding experiences between Annie and her mother is the special bath that Mom prepares for them to ward off the bad spirits. Annie takes a full bath every morning and a sponge bath at night. Annie's mother even prepares special bath water with bark for her husband that she leaves over night so it will be cold with dew.
The whole family is squeaky-clean, because her mother seriously believes that cleanliness is next to Godliness. No, really: this hygiene-consciousness and these spirit-repelling baths keep the John clan free from spiritual danger.
This obsession with bathing (which, to be fair, is one of the worlds best and most relaxing activities) definitely makes a mark on young Annie. When she has a super-high fever she sees the photographs of her family "dancing"—she's totally off her head and hallucinating—and decides that the best way to take care of these creepy dancing photos is to wash them:
I washed [the family photographs] thoroughly with soap and water, digging into all the crevices, trying, with not much success, to straighten out the creases in Aunt Mary's veil, trying, with not much success, to remove the dirt from the front of my father's trousers. (7.15)
This is Annie's illness-addled brain making symbolic loop-de-loops. She knows that there is something amiss with her family. Her parents and grandmother are worried sick about her being sick, and nothing between her and her mother has been right since her adolescence began. So she turns to the method of "making things right" that's she's been brought up with. If it's broke don't fix it—bathe it.
She's actually being really sweet and loving at the same time that she's being a total weirdo. She's trying to make everything right. But she ends up destroying family mementos: oops.
But hey, who hasn't done insane things when they've had high fevers or, even more hilariously, after their wisdom teeth have been pulled?