Don't know if you could call it a journal, but I keep about a dozen index cards and a couple of Fisher Space Pens in the BOB. Might want to leave notes, or record important happenings as others have mentioned above.

I also keep a log on the sailboat. That way I can check and see if my trip estimate (distance, time, currents etc.) comes out anything close to the real world. It's embarassing to take a couple of guests out for an overnighter and find yourself 25 miles from home as the sun sets on the second day.