What sets a hot tub apart from a simple bathtub is its water jets. First used on the original Jacuzzi tubs in 1968, the standard water jet pushes water in a straight line so it hits a single spot at a steady speed. Modern tubs offer a variety of options. Swirl jets move water in a circular pattern and moving massage jets oscillate the water along a line. Shoulder jets installed above the water line direct a stream against the shoulders, and they may be paired with pillow jets that massage the neck. Mini jets can be clustered to create a multi-action massage against a specific area.
When choosing a hot tub, don't assume that more jets are always better. Experts say it's far more important for the jets to be placed where you'll find them most comfortable, and that varies from person to person. Some people like a powerful massage while others prefer a gentle swirl of water around tired muscles. The best way to find the right setup, is to do a "wet test" of any hot tub before you buy. Many spa retailers will allow you to test the spas in their showroom; however, if you're buying online or from a department store you'll just have to take your chances unless you're already familiar with the model you're interested in. If you are buying from a specialty retailer, you may be able to customize the location of jets in a higher-end tub to meet your particular needs, but even jets in a standardized placement might be adjustable. This would allow you to switch individual jets on or off or adjust their flow to create your ideal massage experience.
Note that inflatable hot tubs and even some very cheap above-ground models might lack water jets altogether. As noted in the discussion of types of hot tubs, these use air blowers to agitate the water in the tub. User reviews of such hot tubs say that these blowers often do a good job of replicating the swirl of water in a standard hot tub, though not the massaging action or other benefits of water jets. The other downside is that we've seen reports that the bubbling action can cool the water down fairly fast -- limiting usage to a half hour or less according to some reports. Some also gripe that the blowers can be noisy.
Seating is another factor to consider. Obviously, the number of people a tub can accommodate will depend on its size: The smallest tubs hold just two people while the largest can fit a party of 10. Seat position and shape are important, as well. For example, the size of the footwell -- the space for feet below the seats -- will affect how crowded the tub feels when full of people. Simple bench seats, often found in traditional wooden hot tubs, are easy to move around on and accommodate larger groups. Molded seats offer more support but may not fit all bodies. Special lounger seats allow you to sprawl full length, but they take up a lot of space. Again, if you can, test the tub in person to make sure the seats are comfortable for you.
A hot tub cover keeps debris out between uses, but its most important benefit is to significantly improve energy efficiency. The cover may come with the tub or be sold separately. Not all hot tub covers are weather resistant, but aftermarket PVC soft covers can be used to protect the insulated cover from the rain and sun. See the discussion on hot tubs and energy use for more about insulated hot tub covers.
Besides the basic arrangement of seats and jets, hot tubs now offer a wide variety of amenities. Some models come with both interior and exterior lighting, multiple pumps and molded headrests. High-end tubs might include built-in sound systems and even TV sets, all controlled from a panel on the tub itself. There are even hot tubs with waterfalls and built-in workout equipment. And, while it might seem mundane by comparison, don't forget about steps to make getting into and out of the hot tub easier and possibly safer. Some hot tubs include those, with others it's an extra-cost option.