I agree with BossEyed that you would be better concentrating on things that are attached to you. The PLB I carry with me is not much bigger than a matchbox, yet has a GPS and is waterproof (and floats). I would also suggest a waterproof/damage-proof cover for your mobile phone, as that would probably be more useful than a handheld radio if you're floating around in the oggin. Take a trip to a boat shop/chandlery or whatever you want to call them and you'll find there are plenty of flares available and floating safety bags, which would be much more flexible than a box in the limited space available. A small first aid kit would be useful to add to your bag and a ziploc-type bag into which goes your wallet, passport, car keys etc. No, they're not vital in a survival situation but, believe, me they become more important once you've been rescued and it's a pain in the proverbial if you lose those sort of things. Don't put them in a nav bag because you almost certainly won't be getting that back if you have to ditch....the safety bag you'll grab when you exit the aircraft. Again, I would agree with BossEyed that an immersion suit is almost essential, especially if this is not going to be a one-off trip. I only say that based on the law of averages but, of course, it could all go wrong on the first cross-water excursion and an immersion suit would be a great bit of kit for any water crossing. As an ex-RN aircraft driver I'd point out that we only got rid of immersion suits once the water temp got above 15 degs.......for both of the days :-)