So is coconut water just more hype and marketing spin?

Maybe.  The full paper is available here.  In terms of hydration it appears to be just another drink.....

Comparison of coconut water and a carbohydrate-electrolyte sport drink on measures of hydration and physical performance in exercise-trained men

Conclusion: All beverages are capable of promoting rehydration. Little difference is noted 3 between the four tested conditions with regard to markers of hydration or exercise performance in a sample of young, healthy men. Additional study inclusive of a more demanding dehydration protocol, as well as a time trial test as the measure of exercise performance, may more specifically determine the efficacy of these beverages on enhancing hydration and performance following dehydrating exercise.

Guar Gum diarrhoea

Late back from work on Wednesday night it felt too late to cook but I wanted something so I opened a can of coconut milk.  Lovely.   Within 15 minutes my gut was bubbling.   In the middle of the night  - 3am - I got up and had an explosive liquid shit.  Fantastic.  Final dribble first thing in the morning......

Interesting listening the next day to the superb  Paleo-Nerd-A-Thon! podcast with Robb Wolf, Matt Lalonde and Chris Kresser,  Matt mentioend that some people struggle to tolerate coconut milk, not due to the milk itself but the guar gum ingredient which is a legume.

Time to check the labels more closely in future.