Irritation of the skin when it gets wet is especially common in people with a history of allergies of any type, as these people tend to have more sensitive skin. Water tends to strip off the skin's layer of protective oils, which leads to redness, rash, dryness, and itching. This is especially aggravated by swimming pools, where the chlorine and other chemicals in the water can directly irritate the skin in addition to removing oils. Hot showers, extensive scrubbing in the shower or long showers, or using harsh soaps can have the same effect.
The first step is prevention. You can do this by reducing the number of times you shower. Many people with this problem should not shower every day, for example. You can also switch to a milder soap, avoid hot and long showers, and upon exiting the shower immediately dry the skin by dabbing (not rubbing) and apply a good skin moisturizer. You should be especially careful in the pool and you should always shower immediately after getting out of the pool to remove the chlorine and other chemicals from your skin.
If these methods do not work for you, talk to your primary care doctor
who might have additional recommendations.