One of the worst scenarios that can happen is you inflate your tube only to find out that it is not holding air. This is even worse when you find this out while you are in the water as you will need to get out as soon as you can. A simple way to avoid this is to inflate your float tube beforehand and to allow it to sit for a couple of hours. Come back and see if any significant air has been released and if it has, you will need to check to see where it is leaking air. If the leak is small enough, a simple patch job should fix the issue.
Also, make sure you have both an air pump and a patch kit in your tube as if you do find out you leak while you are in the water, this will allow you to find dry land and perform a quick patch. Then, you can manually re-inflate your tube to working order.