I'm not sure about optimum, but I keep mine at 40-50% without mold issues.

Just a glass of water won't help, but a large pan of water might if it's warm enough. You can humidify your room putting a tray of water over your ballast, which is always warm, but that's a huge shock risk so I don't recommend it. Last winter I put a tray of water in front of my space heater's blower as a makeshift humidfier. It seemed to work fine.