Jbibb;
If I recall correctly there are a few pulp mills up your way. Where there are pulp mills there are jobbing shops that work with stainless. It doesn't look like a complicated fabrication, especially if you have the old piece to pattern after. If you can find a jobber to do it, specify 316/L stainless. 304 does not have any molybdenum in it and less nickel and does not do well in stagnant salt water. There are exotic stainless steels that are almost impervious to the chlorides in the salt but they are expensive, and in your neck of the woods probably rare as hens teeth. I would be a little nervous of using aluminum in that application considering there are fasteners in direct contact with the aluminum and any salt water in the bilge and you have created a voltaic cell, with aluminum being the sacrificial anode. If you have to use aluminum, use one of the 50 series marine alloys like 5056 or 5083 although the later does not do well if forming is required. Try to insulate it from any dissimilar metal fasteners etc.
Last choice might be to use carbon steel like the original and have it galvanised, or give it copious quantities of cold galvanizing coating, or seal it completely in
epoxy as suggested above.
Sometimes I think it was a shame that the designer used that beam but he was trying to achieve a goal and that seemed like the thing to do at the time. After all the boat is no spring chicken and it has lasted up till a few years ago.
Feetup
P.S. If you are down near Vancouver for your course you could pick up a few pieces of stainless and a little welding rod and have it sent to your welder in AK. Or, if you could bring the piece, or a drawing down you could have one made down here and have it shipped up. The shop I work for is near where you are going and we do stuff like that often.