Kinda scary! Sounds like you caught it in time though.
Not sure of your exact setup, but I stuck brand new GLM aluminum manifolds & risers on the boat a couple of years ago. Before I installed them I checked all of the mating surfaces for flatness and a couple of them were pretty far out, and all of them needed attention. I used a sharpie, a steel ruler, and a good file to get them dead flat. Took about an hour. The aluminum really expands & contracts with heat cycles, dramatically more so than cast iron, so I wanted to make sure they were good and flat to begin with.
Also, the hardware loosened up appreciably and needed to be retorqued to spec after the initial run on the hose.
I know the current exhaust TSB from Mercruiser says to install the graphite riser gaskets dry, but I use a tiny coat of Permatex aviation gasket sealant on all of the mating surfaces & hardware and it makes me feel better.
Curious to hear what you find. Good luck!
"Speed's fine, but accuracy is final."
- Bill Jordan
1998 Donzi 21LXR
1971 Boston Whaler 13