Shower heads are a pretty popular way to masturbate.
Keeping the spray out of the vagina is important because the water is pressurized and could conceivably force air up through the cervix, into the uterus and from there into a blood vessel. This can cause an air embolism, which could interfere with the heartbeat. This is more likely to occur during pregnancy. The spray could also force air into the abdomen, causing cramping. It is not actually possible to wash the womb or uterus.