Corrosion of low-carbon steel in salty seawater is about 0.005 inch per year for the first 8 years, after which it decreases significantly. The rate of corrosion is not affected by increasing the carbon percentage of steel. However saltwater corrosion can be slowed and virtually eliminated by using an organic coating, and by using solar powered cathodic protection. According to Stelco the cheapest low-carbon steel rod can be purchased for $500 Canadian funds per tonne (up to 5/8" dia). I guess the challenge in designing a tension-wire floating raft would be to draw a comparison between the cost of steel per acre (including replacement costs) and the cost per acre of regular farmland, i.e. preparation for planting, fertilization, irrigation, pesticides, herbicides, soil ammendment, farm equipment financing, years in fallow, etc.