I feel your pain. I haven't lost anything like that for a long time, but remember the feeling of violation.
I had someone take my (20 year old) kickboard during a lap swim session a couple of months ago. The facility has kickboards to use, but mine apparently looked better, or was closer. I found it a couple of lanes away as I was leaving, and could only toss their shoes in the drink as retribution, since I didn't know which person actually took the board.