I'm not a doctor, but I do know a lot about medications and how they impact body systems. I'm not diagnosing you, no one can diagnose on the internet, so I suggest you speak with a doctor about this ASAP!
Invega Sustenna can increase prolactin levels.
Prolactin is a sex hormone. In men, if these levels are elevated, it can lead to decreased testosterone levels. This in turn can lead to decreased sperm count, which could explain why you are experiencing watery semen. Low testosterone levels can also lead to a decreased sex drive, which can explain why you are not experiencing much pleasure from orgasm.
I suggest speaking with your doctor about this immediately. Low testosterone levels are something that can be treated. If you leave it untreated for too long then it can lead to numerous health problems. They'll probably run some tests to see what your hormone levels are.
Hope everything works out for you.