Just enough

A bonefish's life revolves around the idea of just enough water.

Enough to cover it's dorsal and tail and so hide it from ospreys (and human beings bearing bits of fur and feather attached to hooks).

But only just enough. The slime on the bonefish's body allow it to travel at speeds that its main predators - sharks and barracuda - cannot match, but only if it stays in water that only just covers it's body.