Having sailed for a couple of months without one when ours packed up a year or two back I'd no, provided that you are sticking to easy bits/ bits you know well. I'm sure that our couzins on the east coast won't agree but let's face it most of the S coast and a fair bit of far bank is buoyed well enough to use mark one eyeball or position finding gizmos.
If you sail mainly in the West country, the hard bits are pretty steep too, so a depth gauge isn't much use other than to confirm that you have indeed hit a rock or the bottom....
Now, being a rufty tufty East coast sailor, I wouldn't venture out without one....
I only really use mine to tell me what depth I'm anchoring in, I use the chart & tide tables for everything else. A downward looking echosounder only tells you what you've already run over, so I've never looked on it as a piece of "safety" gear, although it might help you if you're really lost.
My reason relates to anchoring up only not navigation - and I agree the reasons given above.
Chart or plotter will only give you the contour lines not the contours and can be most misleading when you are trying to find a nice sheltered spot. Knowing the exact depth of the area you are in goes hand in hand with tide tables etc to work out what it will be and when but you either need the tarting point (actual depth) or will find yourself leaving significant margins for error, or sleeping on a slope occasionally!
When sailing upwind I use it to decide when to tack. Where I sail (Belgian coastal waters and Zeeuwse Stromen) the length of the the tack is determined by the breadth of the waterway. If you cannot see the depth, you are bound by the channel markings. With the depth sounder on, you can make far longer tacks, thereby critically reducing the beat.
Not sure about the word 'essential', but I'd say it's much more useful, bordering on essential, than most posters so far admit.
Occasionally, by giving readings that were not expected for whatever reason, it has drawn my attention to a situation while my attention was distracted (daydreaming). GPS is definitely not enough on its own - not so much because of any reliability concerns, more because it is very easy indeed to misread a figure on the GPS (I did it once by one degree longitude, placing my position 40-odd miles from the true position... /forums/images/graemlins/blush.gif) especially when you're tired. Also, it's easy to make a mistake when transferring GPS plot to the chart.
Misreading a depth figure in metres requires far more skill.
Often while navigating I am thinking - 'if the depth drops below [say] 4m, then something is wrong'. Sometimes it does, and sometimes I am wrong.
Must admit I'm of the same school of thought. Admittedly it is not essential if steering "battleship courses" but I certainly follow contours when going round headlands .. I'd rather keep an eye on the depth & have a paper chart in my hand than switch on the chartplotter.
once met an old geezer in a Nic 32 with charts where depth was in fathoms, no gps just a sounder who sailed from Tarbet in thick fog to Campbeltown following 4 fathom depth and got there before us ..... so, yes, a depth sounder is useful for some esp in scotland ...
Not essential for anchoring - you pay out the chain & warp till you feel it hit the bottom - if you can't feel it hit, it is too deep or your just not sensitive enough! /forums/images/graemlins/tongue.gif
Then - if you've marked off your rode in suitable sections (ours is every 5m with a nice easy code to understand) it is quick to pay out the required depth. That together with 2 or 3point fix on the shore will sort you out...
What you do in fog though ... well - I still recon that if your depth sounder is bust, you put your head in the water and shout PING... keep your head in until you hear the return.... it won't matter what the depth is then! /forums/images/graemlins/tongue.gif
There's 17 ft when on the mooring - come high or low water. Thats enough for anyone.
Battleship courses on the laptop seems to suffice - I do find it difficult counting how much chain I've put down and feeling for it to go light - 10mm chain never does seem to. So when anchoring I think one is pretty useful
Anyway - its one of my favourite anchorage relationship development ploys - "I say old chap - how much water do you have - damned asdic is on the blink"
Never seems to work with Sadler sailors though.