water is not the problem; it's what's in it. Water containing salt, chlorine or bacteria can be forced into the vagina by the thrusting motion of sex; this could lead to irritation or infection.
Yes
At first glance it may be a sphincter issue, so consult a doctor.
You can if the person you are having sex with has a STD.
It's not possible to "catch" endometriosis - from anything!
It is possible, but not likely. The semen would need to get into the vagina to inseminate a woman.