What is this device on my water heater intake?
I have not found an exact match for it, but I'm fairly sure it's a backflow preventer / siphon breaker / vacuum breaker.
Nothing else makes sense with that low of a pressure marking, and I have found similar examples, just not this exact model.
The intent would be to prevent water siphoning back out of the water heater into the cold water line in the event of a pressure loss on the cold water line. Or perhaps preventing siphoning to the hot water outlets is the intent, per this section of the UPC:
UPC 608.7 Vacuum Relief Valves.
Where a hot-water storage tank or an indirect water heater is located at an elevation above the fixture outlets in the hot-water system, a vacuum relief valve that is in accordance with CSA Z21.22 shall be installed on the storage tank or heater.
The IPC also wants them:
504.1 Antisiphon Devices
An approved means, such as a cold water &quot;dip&quot; tube with a hole at the top or a vacuum relief valve installed in the cold water supply line above the top of the heater or tank, shall be provided to prevent siphoning of any storage water heater or tank.
504.2 Vacuum Relief Valve
Bottom fed water heaters and bottom fed tanks connected to water heaters shall have a vacuum relief valve installed. The vacuum relief valve shall comply with ANSI Z21.22.
Among the various reasons for that would be preventing the water level from falling low enough to allow the heating means to be exposed above waterline, resulting in damage to the electric element, or tank itself in the case of fuel-fired heaters.