Bear, your answer is both right and wrong. Salt water being denser will always settle out at the bottom and fresh water will float on it like many wells in San Pedro.
Now if you contain this water the salt water will eventually "homogenize". Stirring and heating the water will make this happen quicker.
Harriette, I'm not sure just how what I just said applies to your application so don't be mad at me if your plants die from using the sea grass.
I don't want to get the finger when I drive by.
ps It seems to me if the sea grass was arranged is such a way that the rain rinsed it or your container was emptied on occasion the fresh water would eventually rinse the salt out of the sea grass. The soaking in fresh water would help. A way to drain the water off the bottom would also help, so as not to reintroduce the salt to the sea grass. I'm kind of winging it here.
ps ps I'll be down soon, wouldn't it be easier if I brought you down some "Miracle Grow" for you?