Originally Posted by TintedChrome
...But I never want to have to do this again.. other than G10/FR-4 fiberglass board (which seems extremely expensive at $1000+ for a 4x8x.750 sheet - if you have sources for cheaper by all means let me know!), is there any material other than wood that I can use that won't rot or otherwise compromise the strength and rigidity of the boat? The lack of the "wood" look is not a concern - there's going to be enough of it elsewhere (and some small cabinetry on the bulkheads) to keep the cabin "warm".
If it were me, I would use marine plywood, and seal it up with a few overcoats of clear epoxy, obviously paying particular attention to the endgrain. If I were feeling good about my time and budget, I would add a step of layering on a sheet of cloth over it to aid in the sealing up. It's pretty tough to beat plywood for cost, strength, etc. It also takes a long time to get really damaged in this situation considering that its horizontal. So if you are addresing the boats other issues, I would think that this would be a one-time repair for you whether using G10 or plywood.
I'm in the middle of a stripped-to-the-hull refit at the moment, and have had some similar decisions, although far less dramatic than yours. I'm pretty happy with the strength and durability of plywood, especially when weighed against the cost of pre-made fiberglass board. I'm only using that stuff for backing plates.