A Secchi disk works like a lead line, except that instead of flinging a lead line overboard, its user simply lowers the disk into the water.
Once the Secchi disk itself sinks out of view, the user checks the markings on the line to see how far down into the water the disk went before it faded from sight. The user takes a second reading when they pull the disk back up, then compares the results.
On this Voyage of Discovery, Jena and Natalie initially chose to measure turbidity in New York Harbor for their student presentation, but ultimately opted to switch topics after a morning of static results (to are to be expected in these conditions). Whenever the ship was stationary, we also checked turbidity levels for the Deck Log.