There are a baziillion things that can be rationalized to put in a ditch bag, but as psneeld implies, how big and how heavy do you want the bag to be?
Where you boat will determine the bag's contents. The post-abandon-ship requirements in the ICW where the shoreline is only a few hundred yards away in most places and help is readily available if not right next to you is a different deal than the shorelines up here where help is often far away (assuming you can contact them), and the forest, heavy undergrowth, and steep slopes can make overland travel extremely difficult if not downright impossible.
So I think it's important to think seriously about the challenges abandoning ship where you boat will present and not get all caught up in cool gear that the blue-water passagemaker will need.
So we have in our bag the essentials for communicating in areas where communication can be difficult--- hence the aircraft VHF---, what we need to find and navigate to the most landing-friendly bit of shoreline in the immediate area, food and water for a few days at most (water bottles, power bars), something to help us rig shelter from the ever-present rain, some basic first aid stuff for bug and spider bites, minor cuts, and so on, and stuff to meet the dog's needs.
All this fits in a spacious but not huge bag that is easy to stow on the boat, easy to carry, and is set up to be instantly clipped to the dinghy so it won't drift away or sink should the dinghy swamp or flip.
There are lots and lots of really clever items of survival gear, much of which can be rationalized for your own ditch bag in the comfort of your armchair. But we have a rule that has served us very well not only for emergency gear like this but even for the stuff we take on a European vacaction or whatever. And that is that if we consider an item and say, "We might need that," we don't take it.
So the contents of a ditch bag are going to be different for a boater on the ICW than those of a boater in Maine/Maritime waters, or southern California, or the PNW, or SFO bay, or the Irish Sea. Some items will be common to all, but most will be unique to the challenges the boater will face in his/her own waters.
Of course an item packed in a crowded ditch bag isn't much good if you don't remember it's there. So we keep a waterproof list of the bag's contents fastened to the inside of the bag next to the zipper.
One carryover we have from the emergency equipment we carry in the float plane is a well-written and illustrated survival manual for the area we boat and fly in. So it has instructions for building various kinds of rain shelters, basic first aid info, what you can eat along our shorelines and what will kill you if you do, and so on. Moreso in a plane than in a boat, the shock of suddenly being thrust into a survival/waiting-to-be-rescued situation can lead to indecision and confusion, so having a manual along that says, "First do this, then do this, then do this," can be a real lifesaver because it can help focus one's thoughts and get the survival process underway until help arrives.