Ill try my best to avoid becoming (too) murky
Regarding the sinking and rising of a body immersed in water, a body will sink almost immediately and continue to sink to the bottom. And when the body is placed in water it will displace a volume of water exactly equal to the volume of the body. For any object to sink when placed in a fluid, it must weigh more than the displaced fluid. For any object to rise in a fluid, it must weigh less than the displaced fluid. A body of constant weight must expand in volume to become buoyant. If the expansion is very slow, the rising will be slow. If the body is attached to a weight of very dense material, the body volume must increase to a volume which will displace a water volume equal to the additional weight. Disregarding the body, if a 50 pound weight is resting on the bottom and it is to be lifted by a balloon, the volume of the balloon must displace more than 50 pounds of water. If water weighs 62.3 pounds per cubic foot, the balloon, filled with only air, must displace slightly more .8 cubic feet of water and must be about 13.8 in diameter. So if the body weight is the same density as water causing it to begin to rise as soon as it expanded only slightly, it would have to expand another .8 cubic feet to lift the 50 pound weight. The Archimedes Rule also applies to the weight. How long all this would take would depend on the rate of generation of gasses within the body.
Trying to cut to the chase
Water flow in a river or lake is highest at the top middle cross sectional area. Flow is almost zero at the bottom or the shore. Flow is not proportional from bottom to top but generally, as a body rises it will start more or less straight up and then the drift rate will increase as the body rises more and more toward the center top of the river or lake.
There are some really disgusting images of dead bodies floating, if anyone wants to look, but it can take a long time before a body with a weight attached rises high enough out of the water to be noticeable.
So, the point:
As someone previously mentioned, it seems reasonable to consider that the Beast, planning to put the bodies in the lake, had a boat, committed the murders, carried/dragged the lighter corpse down the walkway to the boat hidden under the canopy, returned for the other body but decided it was too much work, decided on surgery, began, got nervous, left with the head, got really nervous, knew the main channel was right there just beyond the cove, attached weight(s) quickly, and slipped the evidence off into the deep water. Over a period of many days, the body drifted downstream while rising very slowly.
The questions:
What possible reason could there be for going several miles downstream to place the body and secure it in position at Riley Shoals if the time required for the body to rise were the same regardless of location and the Beast was under the impression that any weight would keep the body at the bottom? What could be so special about Riley Shoals?
If Sheriff Sills would acknowledge that there was a weight attached to the body, would that prove positively that the Beast used a boat? If the public knew the Beast used a boat, might someone remember something?