Concrete filled tyre is the way to go, if you are at Muddiford as I was , at low water you can dig a hole to bury it , it held a 20' day sailor quite happly until some one else found a use for it at low water and removed it,
leaving my boat to float up on to a sand bar, could have been worse it could have gone out to see, I was told that this does happen around there so beware.