Living near the lake I have seen and help many here with the same problem and have found that most of there floors and supports are rooted. I have taken out more plastic sheets and pulled off all kinds of insulation that has some kind of fungus growing on it. We have found that plastic stopped the water from evaporating under the house. We found that putting plastic under the floor rafters was bad as it held moisture against the wood. Here is what we have been doing.Put a good seal coat on all wood, 5/8ths 4'x8' pressed insulation board black in color cut to fit in-between floor joints, You might find a better product.Trench around the house and place in black drain piping. Just a point we take a hose and make a small flood at different locations to see where the water runs to. Be careful using sealers, make sure to vent and ware proper safety gear. Cement poured did not help all the time and the cost was very high. Cost for the above was not bad and seems to be working for the last three years. By the way gas heat dries the air and heat rises up so your floors will be colder then by the ceiling. We live on a slab with ceiling vents. Consider moving your ducts under the house. I am moving my vents down to the floor by the walls.