Tea, cofffe, diet drinks, soda pop, juice, milk---they all hydrate the body and can be counted as part of your water intake. Of course each type has an upper limit regarding health and this SP article gives the amounts: www.sparkpeople.com/resource/nutrition_art
Regarding your tea, regular and decafe---it does hydrate similarly as water does. The effect of the caffeine is so minimal. ENJOY!
Hope this helps
SP Registered Dietitian Becky