Light scattering can be estimated from horizontal visibility at not too high relative humidities. bscat~3.9/VIS (VIS in meters). the factor 3.9 is sometimes taken as 2.9; it is the Koschmieder parameter. You should google YOURSELF for this parameter

In my opinion, sea grass is not a good raw material for liquid fertilizer. The nutrient content of this plant is too low to qualify as a fertilizer. I am not aware of research on the use of extract of this plant as a plant biostimulant (which is quite different from fertilizer). 

I think you can extract from seagrass some hormone. because in my opinion many hormon in shoots area. and that hormone can apply in crop