For those concerned about maintaining their body's serum (blood) electrolyte levels in the tropics:
Fresh, unsweetened coconut water is considered by many to be the best natural supplement you can find here, and it's pretty cheap in local fresh markets and beverage stands/shops.
Green coconuts have less sugar in the water (we buy these).
Brown coconuts are older (riper) and the water is sweeter.
My wife buys ours fresh at a local coffee shop near the fresh market.
They put the liquid in a sealed plastic bag for you and will include some of the fresh and moist coconut flesh, unless you don't want it.
It keeps in the refrigerator for 1 or 2 days, then my wife discards any unused, but you could probably still cook with it for another day or two (some people prepare oatmeal in simmering coconut water).
We also freeze it fresh into ice cubes and it makes my black coffee (cà phê đen không đường) very delicious without adding any sweetener.
My wife pays ₫20k for one coconut at that Cẩm Châu, Hội An coffee shop; sometimes as much as ₫22k in the market.
I don't think they bump up the price for foreigners, but probably helps to offer the exact change, showing you know the price.
I'm pretty sure if you purchase in a regular tourist area (such as Bến Thành Market in HCMC) the price may be doubled.
Here's a good info site for that and other options:
