Back flow preventer

I want to change the shower sump in a 1999 330 Fiesta Vee while it is in the water. Is there a back flow preventer in the gray water, thru the hull discharge hose? I don't want to flood the boat. This is my first time on this forum. What is the meaning of "Tags"?
Comments
2018 Cherokee 39RL Land Yacht (Sorry...)
I did replace the thru hull with a right angle brass fitting and replaced the junk plastic tubing with better stuff (hose kept leaking). In the process I ran the new tubing with a loop to prevent water from flowing backwards in high seas. More a CYA since I had it all apart.
PC BYC, Holland, MI