I'd say let need determine how and what you measure.
I've set up and tweaked quite a few lights only by measuring at/above the waterline. It has been sufficient.
That said, it's not hard to put your meter in a ziplock bag (spring for name brand and high end) to get measurements underwater....it has been done. :)
Some of the other brands mentioned make waterproof meters, BTW....costs extra, but then it should. :)
