DaveD -- yes, Wildco has lots of good stuff, but all for TOP prices. I guess we're a low quantity profession, eh?
I'll let Bruce tackle the pickiness question a little more. Honestly, I've worked with both home-made and commercial Secchi disks all my life. My favorite was when one of my technicians cut one out of plywood, perfectly to Theo's specifications. He painted it black and white, dried it, painted it again, etc. Unfortunately, when he first tried it without a weight, the readings were always zero.
Yes, Secchi readings vary a lot with wind, waves, time of day, etc. However, it still remains a simple, highly useful tool despite that variability.
