Time for you to talk to your doctor about medications to help bring your BP up...Midodrine and Florinef are first line treatments for dysautonomia.

You could also try coconut water (NOT juice or milk)...it is well known for best hydration fluid. Works very fast too. They have flavors (lemon & pineapple) if you don't like the plain.

Plain water is not the best option as it doesn't absorb as well as other fluids...and it runs through you too fast.