Masturbation is pretty normal. If you are trying to impregnate a woman and she's not able to get pregnant within a years time (the average it takes for most couples), then the doctor may say to masturbate less for the sperm count (same reason they tell couples trying to have a baby to have sex every other day rather than every day). But that's not your situation so wouldn't worry about masturbation. Check into the possible causes AuntJessie gives and talk to a doctor. As she said, you don't even need to mention masturbation but just what your visual description is of your semen.
Just to say, you really wouldn't "see" infertility. That's more an issue of the strength of your sperm. But otherwise, read the above.
Masturbation wouldn't cause any of that, but other things could. You might have a hormone imbalance, for example, but you need to see your doctor. Any time you have unexplained weight loss, that needs to be investigated.
Just tell the doctor that you've lost weight (and how much you've lost in whatever period of time) and that your semen is watery. You don't need to mention masturbation, or how frequently, etc. It doesn't harm you.
Let us know what the doctor says.