It is true that caffeinated drinks can contribute to your daily fluid requirement.
Drinking caffeine-containing beverages as part of a normal lifestyle doesn't cause fluid loss in excess of the volume ingested. While caffeinated drinks may have a mild diuretic effect — meaning that they may cause the need to urinate — they don't appear to increase the risk of dehydration.
Still, caffeinated drinks can cause headaches and insomnia in some people. Also, some studies suggest that if you're pregnant, high levels of caffeine consumption could increase your chance of preterm birth or miscarriage.
Water is probably your best bet to stay hydrated. It's calorie-free, caffeine-free, inexpensive and readily available.