I don't recall seeing a depth rating on a Rotary watch, however with the "Dolphin Standard" or whatever they call it and the screw down crown I'm sure it'd be good for 100m.
Water PROOF rather than water RESISTANT on the back suggests it's better than most of the cheaper stuff, and I know they guarantee their watches against water ingress, or at least they used to. In fact if it has a screw down crown I'd be almost certain it's 100m minimum.