While no immersion, I would guess just the atmosphere and sea vapor would kill that brass in little time.

I have aftermarket "catbird" seats on my Orion... I was rather surprised a couple years ago when the aluminum post that makes the third leg for these seats, disintegrated, right at the mount. The "mount" is a SS flat piece made to hold rail tubing... so I bought a section of ss rail tubing and a large tubing cutter and made a new third leg.

I think there might be enough splash and moisture even aft to severely degrade brass.

BTW my "catbird seats" are thru bolted to 2 rail clamps each, and have that "third leg" that makes them fairly robust.

My seats work something like that shown in the image below... except I don't have that angle rail... I am rail clamped right to the rail itself... but do take note of the post sticking out of the bottom of the seat.

All this is bolted together with flat head SS thru screws, thru the starboard.