Some 380 million years ago, a few pioneering vertebrates first made the leap from water to land. And today, tens of millions of their human descendants seek summer amusement by leaping the other way. According to the travel industry, close to 90 percent of vacationers choose as their holiday destination an ocean, lake or other scenic body of water.

We may have lungs rather than gills, and the weaker swimmers among us may be perfectly capable of drowning in anything deeper than a bathtub, yet still we feel the primal tug of the tide. Consciously or otherwise, we know we’re really all wet.

As fetuses, we gestate in bags of water. As adults, we are bags of water: roughly 60 percent of our body weight comes from water, the fluidic equivalent of 45 quarts. Our cells need water to operate, and because we lose traces of our internal stores with every sweat we break, every breath and excretion we out-take, we must constantly consume more water, or we will die in three days.

Thirstiness is a universal hallmark of life. Sure, camels can forgo drinking water for five or six months and desert tortoises for that many years, and some bacterial and plant spores seem able to survive for centuries in a state of dehydrated, suspended animation. Yet sooner or later, if an organism plans to move, eat or multiply, it must find a solution of the aqueous kind.