It's fun, it's great exercise and you won't drown if you just keep your head above water. Warm water training, or Hydrotherapy as it's sometimes called, will make your muscles smile and reduce aches and pains.
A study done by scientists in Ireland in 2008 to examine the benefits of hydrotherapy for treating fibromyalgia states that "Positive outcomes were reported for pain, health-status and tender point count." A similar study was done in Spain the same year which found that "Patients (that had participated in an eight month test with hydrotherapy) had reduced pain, as well as increased strength, flexibility and endurance. They also had improved sleep, better mood and increased confidence in being able to perform their daily activities."
Hydrotherapy is one of the best forms of exercise if you suffer from rheumatism, and because water has 12 times the resistance of air, water training is great for building muscle. At the same time as you're targeting specific muscle groups your small stabilizing muscles get a great work-out just keeping you balanced upright in the water, and the water supports the body's weight, reducing strain on joints and minimizing pain.
But be careful with heat if you have active inflammation, because it can increase your symptoms.
In Norway my rheumatology hospital organized warm water training for the patients who were well enough to train twice a week. The water was around 34 degrees celsius, so just below body temperature, and there was a physiotherapist who held the class and showed us what to do. It was quite fun, but didn't really feel like I was exerting myself very much - until I got out of the water... then I felt how much I'd been training. After a few weeks I found that I was getting noticably stronger and had less pain.