You'll be hooking up the inflatable to the back of your boat with a tow rope or tow harness, so make sure before you check out that you think about these accessories. No use getting out on the water without a way to attach the toy to your boat. So, think of something to hook up with, and safety equipment like life vests and water helmets.
When you go to buy a towable raft the first thing you'll notice is just how much selection there really is, it will take you forever to go through all of this. We can narrow the selection down by what will work for your needs. So, the first question then is, what are your needs? For instance, how many riders do you have? There are tubes made for one rider, two, three...up to a dozen. They come in different styles and shapes to accommodate the differing number of riders, for instance you can ride a typical circle shaped tube, something more raft shaped, or a giant hot dog shaped (or even a hot dog styled) tube. Obviously, there is something for everyone.
Once you've narrowed down riders you can look for your personal preferences, like style, colors, and brand names. If you're a brand shopper some of the most popular brands are SportsStuff, Kwik Tek, and Aquaglide.
There's a lot to consider here when trying to pick the right one for you, but to buy a towable raft you can save yourself a lot of time and trouble by narrowing things down by your own needs and going from there.