The new edition on the Certified Pool Operator (CPO) manual (2012) published by the National Swimming Pool foundation states on p. 179 “One Rule that should be posted and enforced is that “hold-your-breath” games or long underwater swims are NOT ALLOWED. Children and teens as well as adults seem to enjoy this type of activity, which can quickly lead to an emergency and death.” They also provide a generic sign. NSPF certifies most of pool operators in the USA if not globally. Please visit the NSPF website for valuable information and teaching tools here.