My exhaust tips sit just below the waterline with external flappers on them. I have internal flappers also, but the flaps are gone on one engine and it sounds slightly louder and deeper
On the inside if the boat the exhaust pipes raise up about 1ft. in elevation to connect to the risers. My question is would I be safe without the internal flappers?

Here is pic of my pipes inside the engine compartment

Hey that eng compt looks familular! I think as long as your above the deck/hull line you would be okay. do you have swim platform?

No platform yet, I was in the process of buying one from extreme marine, but we are having a hard time finding places to mount it to As you know the transom is really cluttered.

I just installed a set on a triple 42 fountain. Center motor was rigged with no flappers at all. Trailer launched boat and filled motor with water. Anyhow used stainless marine in line flappers. They slid into 4'' exhaust hose and clamped from outside. Easy install if you can reach the exhaust hose. It was hard on this boat, not much room with three 500hp. cost about $130 total with new exhaust hose.
Good luck Michael