Far better than a portable radio will be a Personal Locator Beacon Beacon with integrated GPS.
Watch out for anything loose in the cockpit, moving in flight or being thrown forward when you impact the water and affecting the stick. If it's not loose, how will you ensure you can release it in a hurry on exit?
You are likely better off considering how any critical kit can be kept on your body rather than in a bag.
Also, consider an immersion suit.