Here's a question for you: what kind of exercise do you think you can do in a hot tub? I'm pretty sure only one comes to mind and most people wouldn't consider it an 'exercise', although it can give those who are out of shape a pretty good workout.
But here's one more thing to add that won't require a partner for maximum enjoyment: cycling. Yes, you read that right. You can now cycle underwater in your hot tub with the Fit-Wet system. It's unconventional, it seems frivolous, and yes, it's expensive for sure.
The main benefit is that the water adds extra resistance when you're pedaling. It also shoots out jets of water that massage your body while you work out. Fit-Wet also boasts that it 'engages all major muscle groups for a full-body workout'--but as some people have pointed out, what about the arms? I don't think it's worth dropping eighteen grand but you be the judge.