First an exhaust is a sealed system

it is.

that can't get water into it,

it can. from and only from the water injection point and from the ocean.

now
it's got a water injection system,

yup, which has to be higher than the waterline or it will flood when the engine
is not running and the seacock is open

and an end open to the sea.

yes, that is why the lift outlet hose from the water lift muffler is supposed
to be at least 12 inches, better yet 18 inches, above the exhaust thru-hull.

what word didn't you understand, oxxy?