I've read several of these "survival at sea" stories and one thing they all have in common is the fact that they end up passing many container ships before they are rescued. Certainly some kind of automated scanning would save lives.
The problem might not be technological. It's possible that the container ships see these people and choose not to save them. The owners and crew of the ship have a monetary incentive not to save people: according to [1] they have to pay a penalty for arriving at a port with people not listed on the crew manifest.
There's also the question of whether or not people need rescuing. Simply because you see somebody adrift, even if they're waving their arms, doesn't mean it's necessarily a true emergency. And now we would make container ships stop and investigate? That doesn't work.
Still, even if the data was simply collected and logged, perhaps others could sift through it for clues about where rescues might need to take place. With the cost of cameras and bandwidth continuing to go down, it wouldn't be a crazy expensive thing to install.
In my mental model, rescues differ from stowaways in that the rescued would likely be glad to pay for any fines incurred whereas that's almost certainly not true of stowaways.
But, as has been said over and over in these threads: it's one thing to make commentary on the Internet and quite another to be there and have to choose.