What is a good water temperature for the laser cutter?

Generally you want to keep the machine between 18 and 20°C. running at higher temperatures can be bad for the machine. If you're running off of a water bucket you'll find it incredibly difficult to maintain temperature I would highly recommend switching to a industrial water chiller. Even with your water chiller keep an eye on it. It may heat up on you on a big job. You could potentially rig a larger reservoir if you are having this difficulty on a regular basis.