If you have a non-GPS tablet, but it does have Bluetooth, you can always get a Bad Elf of Dual Bluetooth GPS "puck" to connect to it.
As far as relying on ziplock baggies for water protection? It's been my experience that all they are good for is keeping the water IN them after they ruin your stuff that you wanted to keep dry.
If your tablet means anything to you, get a Lifeproof case. I've had guys out on kayak trips and classes show up with a battered ziplock sandwich bag. Good thing they didn't capsize... Maybe I'd use a double zip freezer bag, but only for splash protection.
Or maybe an aLoksak bag. http://www.loksak.com/products/aloksak
As far as relying on ziplock baggies for water protection? It's been my experience that all they are good for is keeping the water IN them after they ruin your stuff that you wanted to keep dry.
Or maybe an aLoksak bag. http://www.loksak.com/products/aloksak