"Also I weighed a cinder block out of water then weighed it completely submerged and it weighed the same, go figure!"
I'd like to know how you accomplished that. I'm exceedingly curious as to how you defeated the law of buoyancy. Briefly, the law of buoyancy says that any object wholly or partially submerged in a fluid is buoyed (pushed in a direction opposite the attraction of gravity) by a force equal to the weight of fluid it displaces. That's why boats float. And that's why giant oil takers can be made out of ferro-cement (reinforced concrete in layman's terms). That's right, concrete boats.
Here is a picture of a 1 gallon paint can, filled completely full of concrete. Suspended in the air, it weighs approximately 20 pounds.
Here is the same can, completely submerged in water. The scale reads approximately 11 pounds.
The can and concrete displace about 1 gallon of water. Water weighs about 8.3 lbs per gallon. 20 - 8.3 = 11.7 (Discrepancies: The can is actually a little over a gallon in external size. When filled with one gallon of paint, the paint is 1/4 to 1/2 inch from the top of the can. When I filled it with concrete, I went all the way to the top, which makes the volume of concrete a little more than a gallon.)
So, once again, anything you use for an anchor is going to be lightened by the volume
of water is displaces.
As for 20 pound weight holding coons in any situation
I've had them drag full size concrete blocks back to the bank, in shallow water on a hard bottom. And those weigh 35 pounds. That said, a 20 pound weight will do in most
The milk jug idea comes up a lot because, as you said, milk jugs are handy. But a milk jug has a couple things a couple things against it. For one the plastic is slick, and will slide easily. And the jugs are rounded and tapered at the top. Not conducive to getting a good grip. You can add "grapple" material to the jugs, but at some point, your weights get clumsy to carry around.
I like the paint can. You can keep the handle on the can, but don't rely on it for a fastening. I buried a few links of chain in the center of the can for a fastening point. By putting the fastening there, the can tends to "upset" when it is pulled on, and the lip of the can digs into the mud even harder.