Gotcha. I was curious how you were using it underwater! What lux readings do you get at the surface of the water?
Just like on the reef....>10,000 lux.
>10,000 is about an overcast day.
100,000 lux is full direct sun.
Corals have had a lo-o-o-o-ng time to get used to that range. As a result, they seem to do well across that range....with the caveat that more light = more stress for them. So yes, they do well across that range, but no it's not a "more is better" thing.
In a nutshell, I would shoot for >30,000 lux peak(s) at the surface. That should not only hit the minimum requirements, but all the other stuff too. If your intentions are to emulate a 10,000K surface reef, then you would want about twice that amount of light.
I never log actually. The only thing I would look for is the max lux. A 50,000 lux meter will limit you if your using high light. And also stop you from playing around outside in the sun (which I highly reccomend). So I do reccomend a 100,000 to 200,000 Lux model.
Milwaukee makes a submersible sensor. $75 or so. There may be others but I haven't found one yet.
Strangely, even though in the pic earlier mine says 50,000 on the scale switch, it doesn't seem to be limited there. Not sure what the limit is.....maybe 100,000 lux.
I don't garden in direct sun where I'd have to measure levels that high, for better and worse, but the meter has been valuable to me inside and out! I definitely have learned to use it more for other things than just reefing! I can keep tropical plants alive inside for once....so I was (e.g.) able to over-winter my pepper plants last year.