I'm pretty much with Lauderboy on this but heavily dependant on where you're sailing.
In the Indian Ocean you're surrounded by a lot of countries have have no semblance of SAR, don't answer radio
calls, don't speak your language if they do, have now way of getting to you and more importantly don't give a toss whether you live or die.
But in most other locations rescue happens pretty quickly (48 hours is not life threatening). And if the equipment that you do keep in your bag to communicate (handheld GPS
, satphone) had decent batteries and the odd spare when the voyage started, they'll almost certainly still be good when the voyage is over. Ensuring that is really not hard. And if you do get one dodgy battery, a) a spare will fix that and b) it will only take down one form of communication. Believing that two bits of kit will simultaneously go down is bordering on serious paranoia.
After comms, the most important thing to me is water and I'll sacrifice most other heavy kit to carry a few litres of extra water. Then it's paperwork (passports, ships registration) and cash.
All other stuff is location dependant - don't weigh your ditch bag down with a survival suit
when you're sailing from Bora Bora to Samoa. So my ditch bag gets assembled using specific voyage logic at the start of each voyage