I spent years going deep sea on large ships and the probability of being seen on a ship's radar is far less than you might believe. Most ships tend to run a 24 mile scale at sea. It allows them to pick up ships 15-20+ miles away which is necessary due to their relative speeds. The consequences are that small boats (under 65' or so) tend not to show up nearly as well until the 8-10 mile range. And the sea return interference can extend out 3-4 miles from the ship (on radar). Often it gets dialed out which also eliminates weak returns. So essentially, you have a window of 4-8 miles to be detected, observed, tracked and avoided. At 16 knots, that's a 15 minute window where you can be seen, less for faster container ships. So if the watch officer is in plotting a fix, making coffee or BS'ing, the potential for catching sight of you on radar is still further reduced.

Now, good weather, i.e. calm seas, increase the detection range somewhat, sometimes you might even show up at greater distances. But in good weather, you will also see the ship coming. In the bad weather, the sea clutter really gets dialed down and you're pretty much on your own. Personally, I believe that you've got a better chance of being detected by AIS or, failing that, detecting the ships via AIS, AND you get much better info (course, speed, ship's name). Given the choice, that's what I would prefer.

I confess I must agree with Dan y's "mental m...." assessment. (Something we tend to do far too much on internet forums, btw) Sure, get a blipper or whatever if it makes you feel better. I'll spend my money on something else and keep the best lookout I can and cross my fingers after that (because there's always a situation that you can't plan enough for--- Japan's recent nuclear issues come to mind)