Ossy,
If they do find him, hide that drum!
I have been too busy the past few months to do much in the the way of writing, but the next couple of months look reasonably free, so I will buckle down any minute now.
I did speak to somebody a couple of years ago who had been part of an expedition to find Drake's coffin. He said that the bottom of the bay was very muddy, but I guess it will vary from place to place. There is a fairly detailed description of his burial and the sinking of the two ships in one of the contemporary accounts, which has been reproduced by the Hakluyt Society.
Best wishes,
Mariner