Heights, speeds and track spacings for a search are based on probability of detection - on one search you are not going to get 100% probability unless you search so thoroughly (slowly) and only cover a tiny area with the time/fuel/daylight available.
If they flew directly over the women then it is not a surprise they didn't see them, you tend to look slightly away from the aircraft when visually searching over the sea.
The probability of detection is markedly increased in good weather, with good contrast between what you are looking for and where it is located.
Searching is hard work and humans miss things - you should have regular breaks when searching visually to refocus your eyes and your concentration but the urge to find someone in difficulty frequently overrides that.