Boat Bench Question

I removed the front bench due to attached water-logged foam, rotted wood bench core, and hornet's nests.

My question is:
What species of wood would have the structural integrity best for this application? (read: minimal flex)

Just under a 5ft span (14' mirrocraft front bench), and the wood will be inside an aluminum sleeve.
I've been leaning towards oak, seal it up, slide it inside the metal sleeve, and bolt it in. Later I will be attaching a enclosed area to add additional structural support/floatation, and storage.

Any input would be much appreciated.

Picture can be seen at bottom of this thread:
https://www.tinboats.net/forum/viewtopic.php?t=16020

My old benched were made from 1" plywood - with a box for bracing - heavy but worked fine

What about that plastic that looks like wood that they use for decks on houses :?:

That will sag, plus it's heavy.