There are many ways to navigate and you can indeed follow a contour, I imagine it is dependent on one's confidence with such or such technique. I remember my dad doing this + dead recknoning in pre GPS/decca time in foggy conditions.... but this was stressful!!! May be it is easier in some places than others. I can imagine it being valid in the Thames estuary where there are big sandbanks which are well defined. You need to be sure on which side of the channel you are! A few years ago we were travelling back from the Netherlands to Brittany in poor visibility off the Belgium coast and we did GPS (it was 100-200m accuracy back then) + fine tuning with sounder. Everything was under control.
If you give me the choice between GPS or sounder, I will say GPS especially if I know that there is a risk of poor visibility. But it is true that a sounder is really useful (especially if the OP fishes on sandbanks) and if it does not blow up his budget that is definitely worth considering. You can find combined units (GPS + map + sounder/fishfinder). A fishfinder is just a fancy sounder.
You can get away with a watch, compass and a log, but this may imply that you wait several hours until the conditions are right to navigate through a tricky area! Each time you add another bit of navigation kit, it just means that you can navigate safely in trickier conditions, seamanship is about knowing what is safe and what is not!
btw: Pancho, top man for volunteering for the RNLI