Cbo, I also sweat a lot and suffer in the heat. There is a well known danger of too much water and little salt (hyponatremia). This can be fatal in extreme cases as you will have heard. I use the zero tablets a lot for training (when don't need energy). I also use nuun which is similar. On ultras I find that energy drinks are difficult to keep down so use these tablets in water to get the hydration and to aid in ingesting energy gels and other food (Washing them down). The other stuff that is great to stop cramps are a product called Succeed tablets. Essentially various salts in a pill form for swallowing. Sounds similare to the h2o product in your link.
All said, don't experiment before or during the race.
Bunged up with a cold now. Hoping to shift it by end of week. 8 mile run yesterday but probably no more running till Sunday.
If you have not done a marathon with a relay before, they are a real pain. You have run 18 miles say, go past a relay station and some bloke with a smile and fresh legs zooms out and cruises past. Trip em up I say! Cold marathon was at North Pole last month. -34 deg C. Ace!
Friends and family viewing is suggested anwhere after Musselborough. If you look at the map, there are several roads that head down from the B1361 ie the road parallel to the coast. Get them to drive as far down one these roads then park and walk down to the coat. When you are running you will notice these 'hotspots' of support that tie in with these access roads. Best avoid the one near the 2nd and 3rd relay station. Sorry cant remember which road this is, but the route map should show it.
I hate running in the heat. Last marathon temperatures I did was sub-zero and perfect! Edinburgh always seems hot at marathon time??