If I put the phone right at the water, close enough to get my fingers wet, I am reading 21,000 lux.
That sounds like how I do it and it sounds like a reasonable reading – possibly accurate.
Like Salty said (again) scan around over the surface in the same manner and see what you get. Remember to angle the sensor to pick up the maximal reading you are able to get from each location. You can put your phone in a zip bag if you wanna take underwater measurements....they seem to come out reasonably accurate.