Compounds of 水心

• 水心
  (n) (1) knowing how to swim
  (2) return favor
• 魚心あれば水心
  (exp) if you do a favor for somebody, you will get a return favor (lit: if fish is friendly toward water, water will be kind to fish, too.)
  mutual back-scratching
• 魚心有れば水心有り
  (exp) (id) Scratch my back and I'll scratch yours