Yes dehydration can cause itchy skin.
When you're dehydrated your skin can become dried out and when your skin is dried out because of dehydration then your skin can become itchy.
Dehydration can also cause illness and death so make sure you're staying hydrated by drinking plenty of water, Juice etc.
Drink hydrating drinks well before you're actually thirsty because dehydration can be very serious and cause several health problems including possible death.
So if you are suffering from itchy skin and have no other skin conditions then it could be that you're dehydrated.
So make sure you drink plenty of water to stay hydrated especially during the summer months when it's really hot.
Use a moisturizing lotion on your dry itchy skin as well to help keep moisture in your skin.