On your list you have a flare gun but no flares.

I would suggest adding:

• PLB
• some ski rope (floating rope)
• duct tape
• sunscreen
• dry bag - purpose built for your cell phone - you need a bag that is dry and will let you use the phone while you are treading water
• a second method to make fire

The ski rope can come in handy - if you have to ditch and you have time, you can tie together all the items that float - like coolers, boat cushions, empty gas tank, empty water jugs, etc. This won't make a raft to sit on but it will make a giant debris field that is more easily spotted from the air. If you don't have time to do that then the rope can at least help you keep everyone together as a group.

I imagine you don't want to spend a fortune on rescue gear since this is a once a year affair but there are lots of other items you could add if you have an unlimited budget. Greatland laser, Radar reflector, strobes, sea dye, handheld VHF transceiver, and quite a few more if you browse around the marine retailers sites.

And a final note - if you ditch and swim to shore, then you're most critical task will be fire making since you'll be soaking wet and shivering and a butane lighter may not work under those conditions. I'd add waterproof matches or a firesteel to your bucket.