First of all the ditch bag should be buoyant (in case you drop it in the water trying to get into the raft). Inside you want some good rescue gear and survival gear.
On a double-handed trip to the Galapagos, we had in our bag:
Waterproof VHF & extra batteries
First aid kit
Handheld RO water maker
Photocopies of ID (passport)
I think we also had a little fishing kit
We also kept a handheld gps nearby. The hope was we'd grab it too.
There's other personal things to consider, like prescription meds, and eyeglasses.
The other trick is to update the bag every year. Batteries go dead, rations & meds expire, etc.