I have a set of gibson slip in mufflers, if i took them out would the rubber flappers be enough to keep water out of the engine? The tips do not have the internal metal flappers......

The imco gatlins i'm putting on my boat will only have the rubber flappers.............

so water reversion shouldn't be an issue, their straight through without the mufflers in

Should not be a problem. I run 4" straight tips right out of the back.