One day last fall, police pulled a body out of Lake Calhoun as we were getting ready to collect samples. The year before, someone was taken out of Cedar Lake while we were out sampling. A body was also pulled from Lake of the Isles, another Twin Cities lake we work on. Newman's urban lake crew must also deal with living people. Fortunately our diver had just gotten out.
Vandalism is probably the most maddening hazard, however.
On Lake Minnetonka we have buoys marking our stations. We noticed a few were missing and set out to replace those when we found that someone had sunk them all. When I was a grad student looking at the biochemistry of retinal pigments, I was on a research cruise removing eyes from deep sea fish and storing them on dry ice to bring back to the lab. I packed the eyes in thermoses in my suitcase but I screwed their tops on too strongly. As the crew hoisted my bag out of the boat, the thermoses blew up my suitcase.