Sitting in a hot tub with jets massaging your body feels good.  It can also be good for you.  Hot tub temperatures are usually 102-104 degrees.  Water that warm relaxes
the muscles.  It also causes your blood vessels to dilate.  This lowers blood pressure. 

Water is buoyant and counteracts gravity.  Thus sitting with your shoulders submerged can make your heart more efficient, at least while you're in the tub. This buoyancy also reduces the day-to-day strain on tired muscles and joints.  Many newer hot tubs are designed with jets that can massage neck, back, feet, calves, and some models even come with "carpal tunnel" jets for massaging the hands of those with computer-related pain from repetitive stress injuries such as typing.

Remember not to use a hot tub if you are pregnant.  If you have problems with your blood pressure, consult a doctor first.