Gordon, insurance for bareboat can be a problem. You might be better chartering someone like Jimbo with his Pacific 22 mid week, which should be competitive and leave you free to carry out the survey whilst he drives the rib. Nice stable platform and has 12 and 24v on board for power too.
There is a photo on the net of a member from RNLI standing on Goose Rock at very low water, check out the Lymington or Yarmouth RNLI pages.
How accurately do you want to survey the wreck? like swim around it? There is also a video (You tube ?) of a round the island yacht race showing a posh yacht striking the Varvassi and coming to a dead stop rather quickly