Hi Ben

Different types of agarose powders have different viscosities in solution as they have different polymer properties, i.e. MW,  branching and purity. There are slight variations between lots of agarose from the same product lines as well. The only way to do what you propose would be to measure the viscosity at varying concentrations whilst keeping the temperature constant. From that curve you can get your answer, but it would only hold true at the temperature you've modelled on.

There is the variation of sea water composition which would also need to be considered, this isn'y my field of expertise but I can imagine this can be more variable than the agarose composition.