Isn't that a Bravo 3 outdrive? In which case it should be thru hub exhaust. Does it have a valve for changing where the exhaust goes? Is the blue tube run after a muffler? Do you have an inside photo?
I have seen exhaust pipes come out the back on high performance boats but typically it is two mufflers and two outlet tips with nothing on them except the flappers. Some can be switched from cool mode to uncool mode. Loud to quiet mode. (I prefer quiet myself.)
If it is an exhaust, why not just put an exhaust flapper on?