First its great that you are watching your monitor. But, what you see may not be about a monitors natural behaviors, but more about what your individual monitor is doing to deal with captivity. Please think about this.
Secondly, you seem to be prejudicing your observations/notes already. That is, your putting labels on your observations. i.e. hes doing this because, such and such. Think about it, your new, the monitors new, what do you know? If your writing down notes, simply write them down without prejudice. Later, after periods of time, you can compare notes and see if they are telling you something.
Try taking notes, then after 90 day periods, look at them compile them. Do that every 90 days. (four seasons, four periods) After a while, you may see something.
Also, what you are seeing is most likely products of captivity, not products of natural behavior. Sleeping in a water dish is commonly a product of a screen top, the monitor is dehydrated and can only conserve water by doing this. Water monitors, normally do not sleep in water in nature. Coming out at night, has little to do with being nocturnal, its more likely an avoidence behavior.(as you noted)
Remember not to confuse your cage with nature or a water dish with what a monitor would really be exposed to in nature(lakes, streams, billibongs, swamps. Monitors in nature do not pick a spot in nature, because it has water, but instead for what the water provides. A little water dish does not provide what nature does.
Remember its common for Savs to sleep in water dishs, yet they are not water monitors. Also, closing the eyes may not be sleeping. Check heartrate.
Keep taking notes and having fun. But please understand, your cage has little to do with nature and how a species reacts to it. Good Luck FR