If you cannot reach the water’s surface safely, you will use the indirect method with the reference point usually being a rail or part of the jetty or rail of a boat.
Lower the disk. When the disk is just below the water’s surface place a peg on the line/ tape measure at the reference point.
Continue lowering the disk until it just disappears from view. Place a peg on the line/ tape measure at this point.
Pull the disk up carefully so that the pegs do not move. The Secchi depth is the distance between the two pegs (i.e. steps 1 and 2), recorded to the nearest centimetre.