As a general rule
: What you have on you is what you will have with you.
How about some floatation? Do you carry a life raft? The ditch kit should have two functions. Use from the water and use in a life raft.
Because it may not be possible to always get to your life raft, you should be prepared to sit in the water until help arrives.
BoutUS did an article a while back where they tested different life vests, both foam vests and inflatables. One of the findings from the testers was how well the belt pack inflatables worked, even in rough seas.
I would recommend placing as many of these belt packs as you normally crew in the ditch bag. They are only 5" x 9" packed, and they will provide 35 lbs. of buoyancy.
You can not have too many signals. I sell a signal kit that carries a ACR ResQLink, Rescue
Laser Flare, Signal Mirror, Whistle, and 20 ft. of type III Nylon Line. All in a 5" x 3" molle pouch that will attach to your belt. This would come in handy even without a ditch bag or life raft.
Water survival is not about how long you can stay alive, it's about how soon you can be rescued.