Make sure you're not taking any medication that may be causing this (e.g. antiandrogens, antidepressants). Try to improve your lifestyle (eat healthy, get some exercise), as well. If the situation persists, go see a doctor.
Excessive masturbation is not wrong or bad or anything like that; it can be a little exhausting though. I would recommend cutting down the frequency of masturbation and avoid porn. Watery semen may mean that you are ejaculating faster than your body can produce the semen (which contains lots of vitamins and nutrients from your body).