We found ourselves, and half a dozen other sailboats, in a 0.5 mile square patch of sunlight surround by fog. Thankfully the AIS showed all the large, fast-moving ferry and ship traffic. There was a ferry doing 40 knots that only became visible 1/3 mile away, which is only 30 seconds at that speed, but we could see it miles away on the AIS and get out of the way. First time I've been really glad I had it.
Made for some great pictures, though!