davejb
Reference the second of your typical incidents. The fishing vessel in question has GPS these days, as does the rescue helo. Therefore the requirement to "find" the vessel, and to "conn" the helo on have largely disappeared. It is almost a case of "nice to have" rather than essential.
With regard to some of your general comments in respect of radar performance, search sweep width, etc. The object of your search either has some form of "comms" with the outside world or not. If they do have a voice link, in this day and age they will almost certainly have a GPS position, and can tell you exactly where they are. Alternatively they will have a rescue beacon you can use to locate them.
If there are no electronic signals (voice or beacon) from them, you might well elect to commence a radar search (base on possibilities of damaged vessel still afloat/wreckage/loss of power for radios). However, how do you know this is the case? With lack of electronic signals the worse case scenario is that they are now in the water (in a dingy or not), in which case the search pattern to adopt would be a visual one. It would be no good adopting a sweep width of say 10nm based on radar performance if you are actually looking for a dingy with a visual detection range of 1nm. Obviously you would investigate radar contacts as you conducted a visual search.
Therefore, a SAR search based on radar performance of the aircraft is not likely. Either the object of the search has some form of electronic comms which the searcher can use, or you can only assume the worst and act accordingly.