I just use my own lungs to fill up my float tube. I also leave it inflated most of the time and throw it in the back of my truck when I go fishing. Unless you're going to be hiking to your fishing spots, or don't have a vehicle big enough to throw the inflated tube in, there's no reason to deflate and re-inflate it. If you really want a pump, the cheapest one you can find should work fine. With my lungs I can inflate it within 10 minutes easily. One tip I'll give you is to leave the backrest inflated all the time. It's tough to get it in and out of the back of the tube, and you can't get it in there inflated anyway, so you'll constantly be fighting with it!
My passion for catching fish is eclipsed only by the fish's passion not to be caught.