There are many solutions for this. The simplest is to use a standard pressure gauge on a vertical pipe sticking out of your pipes. It will be exposed to minimal water vapour and unless you are using a fancy delicate gauge you will be fine. Below is something I found on how gauges work and why you will be fine if water does get into the gauge:
Pressure gauges commonly use a Bourdon tube. Bourdon tubes are a coiled metal tube and as the pressure increases, it forces the coil straight which then moves the needle. (think blowing a party horn, which causes it to unroll) The type of fluid (water/air) makes no difference to the tube, only the pressure exerted against the tube does. And since most of the tubes are made from bronze, copper or stainless water will not corrode it any faster than moist air from the compressor.
In all said, the air compressor gauge will most likely work without any issues.
But if you really want to get a water gauge, go to a pool supply store. They have pressure gauges for filters that are designed for water and use a 1/4 npt. They also have a nifty little ring on the outside where you can mark your normal pressure so it is easy to see if your pressure is low or high.
Pipelines that are drained and filled often have surges of water that can damage piping and gauges by what is called 'water hammer' when a slug of water is propelled at high velocity by air behind it. If your pipe is designed to fill and drain it should be able to handle these surges but I would monitor the gauge for a few cycles to ensure excessive pressures are not observed.