Cork, foam, cedar, the closer to the actual density of a duck, the more realistic it will ride in the water. Plastics "bob" because they are so light. It is a trade off for weight to realistic floatation. If there isn't any wave action, the birds won't notice a plastic bobbing in the water. If you have a 2 pound duck, another object of the same proportions and weight will float similiar in the water, hence the heavier decoys
Cedar blocks are used for diver rigs because they are inexpensive and can be mass produced. Their color patterns could be painted by any one and ducks will flock into them. On the same turn they have the approximate density of a real duck. If you have nice looking painted birds, put them on the outside of the rig. Everyone has experiences with anything working as a decoy, from plastic bleach bottles to a hand carved award winning decoys. For diver hunting it is numbers, some color, set-up of the decoys, conceilment of the boat, and most importantly conceilment of the hunter and their face(s).
From personnal experience, I'm a believer in how a decoy rides vs. a nice factory paint job.