knowing how to swim
Refers to the ability or knowledge of swimming. Often used in contexts where one's swimming ability is relevant.
He knows how to swim, so he's safe even in the ocean.
kindness begets kindness; you scratch my back and I'll scratch yours
Proverbial expression, abbreviated from 魚心あれば水心. Implies mutual goodwill or reciprocal favors.
See also: 魚心あれば水心
There is a proverb: 'If the water has a mind, the fish have a mind too' (kindness begets kindness).