Some lakes and rivers are very clear, while others are very murky with sediment or organic material. Water clarity can yield information about what kind of environment is present around the water body (in its watershed). My local lake is fairly murky, due to significant nitrogen and phosphorous in the run-off from the many well-tended lawns in the area.
Secchi disks, are 20-30 cm diameter disks, generally white (freshwater disks generally have two black quadrants on them as well). These disks have a line attached to their center, and are lowered down into the water until they are just barely visible. That depth is the Secchi depth, and would be recorded.
In the Boundary Waters of northern Minnesota, scientists are interested in how the water quality of the lakes are changing. If you’re headed up there on a canoe trip, you can volunteer to take secchi depth measurements.
Getting secchi depth measurements in the lagoons and near-shore waters of Heard Island could be an interesting project, too.