Nonhygienic behavior likely contributed to three recreational waterborne illness (RWI) outbreaks at Idaho splash parks. The study described in this article examined the influence of signage and hygiene attendant presence on rates of nonhygienic behavior among children at splash parks and knowledge and attitudes of their adult supervisors. Investigators observed children for nonhygienic behaviors at four Idaho splash parks, two with signage and attendants. Supervisors were surveyed (N = 551) using an eight-item survey. Individually observed children (N = 145) were often seen exposing their buttocks to splash feature water and placing an open mouth to water. The rate of nonhygienic behaviors was not lower at parks with signage or staff. Supervisors reported bathing children before splash park entry infrequently. Signage and hygiene attendants do not adequately limit nonhygienic behaviors at splash parks, and supervisors have insufficient understanding of RWI. These findings have implications for developing splash park regulations and RWI prevention efforts.