As Avalon said float tubes and moving water are not good mixture. They are really not stable enough for river use, maybe in a slow stream where you are just using them for transporation you get by.

Don't know the MD regs but in PA public waters you can only use tubes in waters that are designated for them. If there is no posting they are not allowed. I found that out the hard way when I got tossed from the lake nearest my house. I haven't found a listing that tells you where you can use one, but I admit I haven't tried all that hard. I also have a pontoon boat and they go about anywhere once you register it.