Why do I get pounding headaches after playing basketball. I am a 26 year old male and am very active. I also always drink plenty of water.?

Headaches. I don't think that hydration is your problem. At your age I would be concerned if you have pounding headaches everytime you play basketball. It may be a simple problem like migraines but I would recommend you check with your primary care physician or a neurologist for an evaluation.
Exertional Headache. This sounds like exertional headache, which can come on with any physical exertion. Many exertional headaches are secondary, meaning caused by another process, and some experts recommend MRI for all patients with truly exertional headaches, to rule out aneurysms and other serious problems. At least you should see a doctor and be examined for any red flags, or for other headache types e.g. Migraine.