Hello J.,
As a teacher, I can sympathize with the no bottle rule during class. It is a distraction, children use them to fidget with, distract themselves, make games with other students, avoid work by drinking and claiming to have to go to the bathroom, and a host of other inventive antics children do with their possessions in class. However, a teacher with good classroom management skills can arrange for more frequent "Water time." I would suggest asking her to allow for more frequent water times that won't disrupt her schedule and see if working with her would help. Obviously, they've seen this before and dismiss you like their jobs aren't in jeopardy. Which they probably aren't. So instead of the power struggle, which your daughter will end up getting the consequences, (this is only kindergarten, she will have this reputation of "difficult parents" for a long time at that school.) Also, ask your daughter to support the teacher's rule until you've had a chance to work out the situation and don't discuss this issue in front of her, she'll just want to defy the teacher and create her own classroom power struggle.
On another note, I haven't heard of any child getting dehydrated at any school in Calif. except for long distance runners on high school track teams on extremely hot days in the valley. Children have lots of opportunities which occur every few hours to drink something, and the classrooms in most schools are air conditioned. If there is a smog and/or heat alert, principals will ask teachers to restrict play by keeping their students in classrooms for lunch, recess and P.E.
Good luck,
Wendy