I'm afraid I can't answer your question Gra. Just wanted to say that coconut water is something I'm really interesting in too. So would love to hear how people get on with this.
I've decide to try this with my 10 year old (undiagnosed). Dehydration is a huge problem for him, although he drinks between 1.5 - 2.0 liters of fluid a day he generally suffers from dehydration (dizzyness/ headaches/disorientation) multiple times per week. As he has developed bad acid reflux (currently taking a PPI daily) sports drinks aren't really an option anymore.
He's been on a GERD diet for a couple of weeks and as part of this I swapped him off our normal drinking water, which is desalinated sea water and has been heavily processed on to crunchy (lots of minerals) alkaline (PH 7.6 - 8.0) mineral waters. Early days but this seems to be having a big impact on the dehydration issue, he only had two days last week when he showed signs of dehydration and none so far this week.
However I'm not sure that drinking mineral water alone is going to be enough to get him through the hot months (we had 179 days over 100F last summer), so am desperate to find a healthy alternative to sports drinks.
From what I've read coconut water has a PH of (5.0 - 5.4) which although not great for acid reflux is similiar to unflavored Pediatric Electolyte mixes and a lot better than sport drinks which are mostly in the 2.4 - 3.5 range. So think it's worth a try.