I believe that the exhaust
manifold is cooled with jacket water on that engine so the antifreeze
should have protected the head, the block and the manifold.
Seawater freezes at 28 deg F, so it surely got a lot colder than that in New England
As others have said check the seawater pump and the heat exchanger for leaks
. It is possible that freezing of the seawater side of the heat exchanger damaged the tubes and they now leak into the freshwater side.
I would do a "radiator" leak test to find out if that happened. You can buy a kit for $20-30 bucks at the auto parts
store and do it yourself.
The raw water heat exchanger and the raw water pump
are the only two components possibly damaged.