One idea would be to just put a layer of polyethylene sheet down in the greenhouse floor, with a drain hole between two slats in your deck. That would keep the wind from blowing up, send the drainage water where you want it to go, and prevent any damage to the deck. You may want to add another layer of some non-slippery fabric, such as "remay" or "weed barrier", to make it easier to walk on and still let the water out. Since the amount of heat lost through a greenhouse floor in the winter is almost insignificant, there is no need for any other insulation.