I have heard from someone else that the camp site that I mentioned is closed now, but there is another scout camp, they allow occasional visitors, further up the creek towards Shalfleet on the left hand side. I have looked at the 2.5″ OS map of IOW and it is marked. Only try to go up and leave this far up the creek at the top of the tide.
My really practical solution to crossing the soft mud is don’t. Make sure that you have landed, stay in the boat or even better stay in the pub until there is enough water.