start with a free app for your smartphone....such as "galactica luxmeter" for IOS...but there are many of them. It uses the camera's light meter to compute a regular lux reading.
Try to get the highest lux reading you can at the water surface and tell us what you get! (Anything from around 20,000 lux up to 80,000 lux is "okay".)
Also, I'd order an "LX-1010B" handheld lux meter (or similar) to have around for later. They usually cost less than $15, delivered. Better readings and safer to use around saltwater than your phone! :)