My new friend Piotr says:

Hello,

I enjoyed talking to you at ISCAS 2008.

I left the stone you gave me at Otter Cove in Monterey Bay, California. This is
a cove near San Francisco with kelp forests, sea lions, seals, and
occasionally, otters. I was at a depth of 5.1 meters when I placed the stone,
so the stone is probably around 5.5 meters deep. A photo is attached.

Other photos from the dive are at:
  http://web.mit.edu/pmitros/www/dive.zip

This is a large (282MB) file. I will take it down in a couple of days. I think
it is probably not worth downloading, since most of the photos are of low
quality (one really needs a better camera for underwater photography). In case
you do not heed my warning, images 4705-4749 are at Otter Cove, and the rest
are from other parts of Monterey Bay. I took panoramic photos directly over
where I left the stone, but they did not come out, since the lens had water
drops on it.