I've always used a small blue tarp under our tents, just make sure the tarp is under the tent completely so rain doesn't get on it and seep between the tarp and tent.
I'm with AKHunter 45 on this one. I use blue tarps too. Make sure you tuck the exposed parts of the tarp under the tent, or it will duct rainfall under your tent. I also roll my tent up in the tarp before I bag it; this way the tarp, rainfly and tent are all ready to go. There's a way to fold / roll the whole setup so it goes in the bag nicely. Some of my tents have lasted many years this way.
You will want to replace the tarp every few seasons, depending on how much you use the tent. Eventually they start to break down and will leak. I like a tarp that keeps my floor dry from seepage under the tent.
An alternative is to use a sheet of thick visqueen instead of the tarp, but I prefer the tarp because it has grommets and is stronger, which means I can use it for other things if I have to in the field.
Finally, when you roll your tent up, you might consider packing the fly separately, if it is not completely dry. This will keep the tent body drier if you are setting up / breaking down for several days in a row, something not uncommon on float hunts.
I picked up a roll of Tyvek for a house project, and have used it to make footprints, and tarps. It is much lighter than blue tarps, waterproof, and very tough.
Thanks for all the suggestions. Now I've go to get myself to the gettin' place and buy 1 or 3 (I need something to cover meat with also) of the "blue" tarps.
If you want a North Face footprint they will make one for you if you call them. I have a TNF Apogee single wall tent that they don't make anymore and was going to do the same but a 6x8 tarp just happens to fit perfectly for it. Just the right size, a little smaller than the tent floor.
I second the Tyvek! I have been using it for years and it is much lighter and easier to pack than the blue tarp. Plus I don't have any hesitation cutting it to exactly fit the tent (cuts down on the size even more. It is very strong to boot.