The answer depends on your age and the type of water exercise you're doing. Small children and older people typically prefer pool temperatures to be a little warmer. Swimmers training for competitions may need cooler water, so they won't overheat.
In general, pool temperatures between 83 F (28.3 C) and 88 F (31.1 C) are usually comfortable for most types of water exercise. Pools that are used for physical therapy typically have warmer water to help relax muscles and make stretching exercises easier.
Water exercise is a good choice for people who have arthritis because the buoyancy of the water helps protect joints from weight-bearing impacts.