Barmouth is quite a shallow harbour - so my sounder alarm is set at 1.5 metres, but I know that I'm still floating with about 20 cms clearance at 0.8m on the depth sounder.
I only beach the boat on sandy stretches that I know - but don't take chances in rocky areas... (the B2X leg doesn't really trim up very much) ......
...apart from the time I landed a film crew on Bardsey - and it was a bit like that Star Wars game flying an X-Wing down the trench on the Death Star...
It's Ok if you know it though...
The best way to set your sounder is to walk the boat into some shallow water, and when the bottom of the skeg is as close to the bottom as you are happy with - set the offset to give reading of zero. Then you know that the reading you are seeing is the depth below your skeg.