On the other hand, if someone were to scrupulously observe the posted prohibition, and swim underwater while inhaling and exhaling as usual, their might be some interesting litigation as a result of that as well.
As this would quickly lead to unconscious breath holding and as the sign says 'No Breath Holding', either consciously or unconsciously, its still a get out of court free card. Also if you notice the symbol under the 'No Breath Holding' text, it clearly shows a no under water swimming sign or would that be no drowning allowed in the pool sign.