Check out aLOKSAK bags. I've successfully tested my Nexus 7 tablet under the shower with one of these bags. For scientific purposes, of course.
For science, I'd put silica gel crystals in it and take it to the shower before I dunk my tablet :)
You could just use a square of cardboard... why use electronics at all :)
Because it won't tell you if there's humidity inside the bag that could potentially harm your device.
Silica gel isn't electronics. It'll change color when it detects moisture.
I don't mean to sound as a luddite, this is an honest question: why would you need a raspberry pi tablet in a kayak trip?
Maybe he needs a cheap display for reviewing photos taken on the trip? Something, he might do some basic editing on? Something durable, not too expensive, that fits his needs.
Just throwing thoughts on a wall, waiting to see, that sticks.
sdoering got most of it right. I also would add backups, it doesn't feel safe keeping all your pictures on cameras sd cards for weeks. Actually, for me it's mostly backups, that's why a dirty cheap rougged tablet would be nice (cheap because I don't do many of these trips).