2 m down to 50 centimeters (margin) below nominal draft (keel not canted hanging down) basically they where following the 10 m contour on an ever changing (uncharted) seabed. Around midday that day the low high water in the middel of the isles was at ca. 13:00 which should add at most about 90 cm that day (moon on the other side of the earth) The sand is hard but the inclination very easy so minimal risk is sanding down paint. Biggest risk is being slowed down so much by dragging over the bottom that you can't overcome pressure to leeward from waves and wind. Before high water flood will push you up the beach ,the good then is depth will increase all the time.. after high water ebb would carry you off, the bad is depth is decreasing all the time. So they definitely where willing and ready (as they should) to offer the port dagger board. Always ready to hoist it up as first glance of dragging or striking, then heel the boat to lift the keel. I do advise to use a dutch side boarder with a (typical) flat 2 inch steel bottom on your first trials.