Hi, Peter.
Much of my sailing is done on a reservoir so “navigation” doesn’t really come into it, however, I rarely sail without my old Garmin76 - switched on to give me an indication of boat speed for sail trimming etc. - and, when looking at the saved tracks afterwards, to wring my hands over the tacking angles! Its a grey scale screen so it runs for ever on a couple of AA cells.
When sailing somewhere on the coast or abroad I always have charts (waterproof, if possible, on a clip board) maybe an Ordinance Survey map of the area, hand bearing compass and binoculars. Foolish not to! If planning a coastal passage I put waypoint into the Garmin and have details such as tide times, bearings, distances to run, etc on a piece of white board fixed in plain sight. If visibility, or anything else, looks like it might be a problem I simply don’t sail!
I have a depth/fish finder (because I love gadgets!) but knowing the depth under your keel is only useful to navigation if you have a good chart, I imagine.
A full blown GPS chart plotter with zoomable charts and all the bells and whistles would be fun – if it could be fixed somewhere close enough to the helmsman to actually use it - but I rather think it would be of limited value. If caught out in nasty conditions as you suggest, you might be too busy to peer at a rain splashed 5” screen! On the other hand, while at Milford Haven this year, on our way to Pembroke Pool, and not sure where to turn, I happened to have my Nuvi 360 car sat nav in the pocket of my day sack, swiched it on under the spray hood and in a few seconds I had my Latitude and Longitude, ready to put into the Garmin 76 . This wasn’t needed as scrolling around in the map view, although hardly intended for navigation, gave me enough detail to work out where I was on the Imray chart. It was so useful in this role that I might buy it a waterproof pouch of its own. It has a data base of pubs and restaurants, too, which you don’t often get on a marine chart plotter! Garmin used to make a car sat nav (with turn by turn voice guidance) that would also take marine Blue Charts on an SD card. I haven’t seen it advertised for some time, though. I think it has been replaced by the GPSMAP 620 which costs about £600, which is, at least, waterproof.