http://ge.ssec.wisc.edu/modis-today/
I usually go here and look to see where the brownish/greenish water turns straight to blue and without fading. I know for sure there will be a salinity change right there, but whether or not if it has a ton a grass you just never know. It worked for me before and we caught tons of triple tail and even some dolphin and ling just 10 miles out of Pecan Island.
I would say the best thing to do though is find a guide service near the area you are trying to go and call and ask them, I'm sure they'd be willing to let you know.