I fainted a lot/a few times in my teens, but never thought much of it. I tell in advance in some situations now, so people/doctors or such wouldn't be surprised, and get VIP treatment, bloodwork done while lying down etc.
I always thought it was due to low blood pressure, and maybe some slight anemia. Lately, I've had a fainting situation and a few 'near-misses' in the garden when I grabbed on to something, might be related to my neck, or just low blood pressure. It did scare me a bit.
Didn't know it could be due to dehydration as well! But if you drink a lot your blood pressure is higher, so it makes sense...
People usually just want to help, yeah, so no fretting! Stand by something that you can grab (not too near loudspeakers - deaf or hyperacusis aren't fun either! - maybe have earplugs with just in case?) and tell people you're with, so they'll look out for you!
A friend who does a lot of sports and got dehydrated while only drinking water before, now carries some salt with wherever she goes. Not too much a fan of the artificial isotonic drinks myself, you can experiment and see what works best though?.