Hey there.
An interesting thread. I used to carry water everywhere as I found it helped a bit when I felt panicky. I still do, but that's just me.
I must say I totally agree with Trev. Drinking water is great, but I think that it only makes you feel better because you think it will. It might be considered a 'safety behaviour' by those from a CBT perspective.
However, it certainly is important to keep yourself healthy, and one of the things you need to do is to drink lots. Mirry, if you're finding it helpful, then keep right on with it
Don't get too bogged down with the whole Sodium thing. Yes - it's best not to eat too much salt. But sodium (like potassium, as you said) is a vital mineral that your body needs in certain quantities. Moderation is always the key.
As for Antihistamines, I would be extremely cautious. I've taken them plenty of times for an allergy, but not convinced they would be good to use long term for something they are not licensed for. I think it really would be best to get medical advice before taking them to help with anxiety problems.