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This may be a silly question, but, can I replace my exhaust flappers (aka shutters) with the boat in the water? Or must it be done out of the water?

I presume the answer is it depends if they are above or below the water line?

I didnt have a tape measure, but I estimated, mine are 1" above the water line.

I have 7.4L MPI in a 1997 233.

I`m assuming this is for thru hull exhaust? If so I don't see why you couldn't. I used to have an inboard with flappers just below the waterline and they didn't really seal against the pipes. As I understand it, the purpose of them is to keep water from flowing back up the exhaust and into the cylinders if the engine shuts off while under way and you get a sudden rush of water against the transom as the boat slows down.

its a bit risky. better do it on a calm day lol. get a 2x4 and span it across your beam, then you can measure down to the water and measure down to the top of the Y pipe.