Limestone media could work for pH adjustment but I dont think it would work as filtration media to retain iron oxide particles. I wonder how it would be to backwash this type of media. I recomend you to remove CO2 first by clasic aeration which will help also for iron oxidation, then use sand...
An easy estimation for the type of emulsion is to take a sample and let it stay for 24 hour. Observe the %volume of oil that separates. 80 to 90% separation implies mechanical emulsion. 65 to 85% implies chemical tendency emulsion and 65% or less implies chemical emulsion. Conduct a Jar test...
The only difference in this case is to consider physical properties of sea water such as specific gravity, viscosity and temperature. Consider Concrete Portland II for basin material and carbon steel protected with epoxy brea for the skimmer. Use the API spec for designing.