I draw 6'3". At a super low tide my sounder reads 5'4" and I am still floating, but obviously I must be very close to grounding. Also, I am tied to the dock, so the swell is not a big factor. Next time you haul you need to measure from the sounder to the bottom of the keel with a plumb line to get the real number for using your sounder. Or, I don't know off hand, but, maybe it can be callibrated for the difference. Kathleen's advice to get a line is a solid and time tested idea.
Great men always have too much sail up. - Christopher Buckley
Vaya con Dios