this is such a great idea check out the story below.
http://www.latimes.com/news/science/la-fi-seafarm10-2008jul10,0,1419955.story

thats awesome. I wonder how the plant itself tastes

I dont know but it can be used for animal feed as well.
There is also the shrimp and fish farming part of it to.

It's a good idea, but the water would evaporate leaving an over salty water that the plants might die from

I was wondering about the evaporation issue too. But then I saw the part of the plan to keep pumping this increasingly saltier water from one purpose to another. I imagine that by the time it hits the magroves it's going to be pretty freaking salty already. After that? Maybe create an inland salt sea like the dead sea. Maybe extract the salt for commercial use. They do evaporation of seawater for that some now, but I wonder about contaminants that would accumulate that way. Or maybe pump this concentrated brine back to the sea.

I'm going to read up on this some more and maybe find out.