Since NEC doesn't "disallow" it, it shouldn't be a problem. We use this common trench set-up often with primary power, secondary power, communications, tv, and gas line all in the same common trench. We use a similar common trench detail that are usually in utility company infrastructure installation manuals, there is usually about a 12" separation between systems. As far as water line, there shouldn't be a problem. Metallic water lines are ground bonded in buildings, and all electric lines underground get wet. Unless the water company or civil engineer in their codes have an issue.
