mwal,
As you stated secchi disk is used to measure water clarity. I was wanting to use it as a starting point for setting bobber depth for crappie fishing. Being retired on a retirement income I don’t have the resources to buy the latest toys on the market.
Why not just make one and meter your own line? Could probably make one for next to nothing? A thin rope with knots tied every couple feet would be simple enough? Just a thought?