I know where you're coming from Patrick. About half of my summertime guitar playing is around riverside campfires. My "river" guitars spend hours traveling in hot vehicles, get stuffed into dry bags for overnight trips on rafts, get played during rainstorms under tarps, etc. Although my acoustics aren't as high-end as your $4K Martin, I still wouldn't want to subject my D-16 or my Blueridge BR-73 to that kind of torture.
For a month-long trip out west and a week on the Middle Fork in Idaho, I bought a $100 Johnson dread naught and re-worked the action a little to make it more friendly. It held up well and did a good job on the river, but like you, I'd like a smaller size guitar to take along in the future. From the experience I've had playing several of them in local music stores, I think a Bristol BM-16 (000 size guitar made by Blueridge) might work out well. For a cheap guitar the ones I've played sounded great, have excellent projection and play easily You can find new ones on ebay for $189 shipped. You might want to check then out.