When we have continuous scanning of the earth by satellites you could put out some tarp that is easy to scan for with software. Instead of laying out rocks to say HELP you lay them out in something that looks like a QR code.
If you recall when that airliner went missing tons of people starting scanning satellite images for debris. The resolution is simply not high enough to detect tiny little things like debri or life rafts.
The picture per day part is really cool - instead of having to look for a life raft/debris, we can look for patterns of abnormality across time. Given how large the ocean is, we have a good sense of the 'normal' distribution of images every day, so maybe we can find patterns like consistently "abnormal" images along a wind/current vector over 7 days. Individual debris may not be visible, but fields of debris may have some characteristics we can recognize.