Conjecture: Humans require both salt and freshwater to maintain health. Ancestral divers at the seashores had abundant salt, but where did they get their freshwater every day? Did they walk over to the nearest river and drink it there? Well, perhaps they did, but it was risky, since large predators would be there waiting for thirsty prey to come by.
Coconuts provide a source of water, and during rainy periods, puddles and upturned shells would capture rainwater and freshwater springs would be available. Another source of water was the metabolic freshwater produced by the body from eating foods in addition to the free water inside fish.
Another likely source is rivers continuously pouring out their freshwater into the sea. The water further down-current (past the river mouth) is far more fresh than normal seawater, being variably brackish (depending on tide height and position, in some areas about the same salinity as blood plasma) so potentially it was sustainably drinkable without resulting in electrolyte imbalance. Too much salt and too little salt are both bad, and same with too much/little water. See this article on marathons, too much freshwater + not enough supplemental salt is dangerous:
The body needs optimal O2 & CO2, but also needs the optimal amount of Na & Cl, I assume as much when swimming and diving as when walking or jogging along the beach especially in sunny warm tropical conditions. It has been suggested that a diver should be hydrated preperatory to a dive, it makes sense a diver should have the right balance of electrolytes as well, for better cellular and myo/hemo/neuro/cyto-globin operation, and this would affect hypercapnea and hypoxia endurance capabilities while diving.
I'm talking here about ancient divers optimizing their dive time and performance while dive-foraging on a daily basis (not emergency situations), replenishing their water balance without entering inland rivers where the tigers, lions, crocs awaited them and other thirsty prey. (Later they figured out how to make dug-out canoes with embers and stone hand-axes and were able to penetrate the rivers far inland along gallery forests in relative safety, presumably with push-pole spears etc.).
By swimming (perhaps on a boogieboard of driftwood) offshore in clear blue seawater, and swimming into the freshwater outflow for a slightly brackish drink, beyond the normal croc zone, I'd think the only large predator would be the bull shark, which will enter large rivers easily but AFAIK doesn't target small freshwater stream outflows. I assume they used a sort of knife-blade-spatula to pry molluscs and crustaceans and perhaps spear slow groundfish and as a weapon against sharks, this tool would take the place of large canine teeth. (Chimps have been seen using sharpened sticks to spear prey in tree hollows, that's not too different from using sharp sticks to spear groundfish or sharks.)
I should note though, that some seas are very high in chemicals which cause other problems even when diluted to a brackish level, eg. Dead Sea is high in Magnesium which causes diarrhea. Of course in modern days, river outflow water would have to be checked for pollution hazards.
[Note: For more info, see also my website at THE-ARC.wikispaces]
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