wrist watches at USMS sanctioned meets

I attended an open water swim competition this past weekend and about 80% of the swimmers were wearing wrist watches. This meet was USMS sanctioned/recognized as all competitors had to be registered USMS members. None of the approximately 550 swimmers were disqualified for wearing wrist watches. I thought timing devices were not allowed but couldn't find wrist watches specifically mentioned in the open-water swimming rules. Could someone clarify this and/or point me in the right direction as to where this is addressed in the rules.