There may come a time where you want to clean off your digital meter (if you go with a digital pool tester). You can do this just fine, but you need to keep it simple. All you should do is take a non-abrasive cloth with some fresh water to get the job done.
Of course, do not expose the digital screen itself to water (at least directly). You could lightly dampen the non-abrasive cloth with fresh water to clean the screen itself (do not use very much water at all).