Using throttle in reverse is highly unlikely to cause this if you you look at the muffler plus increased exhaust pressure when that is occurring. I would enlarge the drain on the bottom port side of your muffler to drain the salt water out of your muffler before you flush the cooling system after use. There is quite a reservoir in these mufflers so a source of back corrosion while the boat is sitting if the system is not flushed for a long period of time......about the only extra thing you can do that I can think of off hand.
Thanks a lot doc I will look into enlarging the drain for sure and flush after each use internally and externally.
Your other threads and replies have been very useful to me also.
I'm not sure how the internal of the muffler is constructed to hold so much water or as to why or if necessary . You may want to remove it and take a look. Also , I'm not sure if its a male female fitting glassed into the muffler or just a threaded screw as I have not taken a close look. You just don't want to have a leak at this fitting is all i'm saying. Theoretically I don't see an issue going bigger just trying to cover all bases.