The density of the human body is just more than water. Adding a relatively small weight is enough to sink the body. However, gases form during decomposition that increase the buoyancy of the body sufficient to bring it to the surface even with considerable weights attached.
Potential pitfalls:
(1) need to transport the body
(2) need to stop on a bridge or pier or to use a boat
(3) need to carry a considerable weight unless the body is cut into pieces (messy)
(4) need to add enough weight to keep the body under water during decomposition
One approach is to invite or take the person out on a boating trip, then kill the person away from shore. This requires premeditation and planning. In addition the person may be seen or may tell someone else about the trip.
Mistakes made by criminals in the past:
(1) not puncturing both sides of the chest wall
(2) dropping the body too close to a beach or a current that leads to a beach
(3) dropping the body too close to where people fish or swim
(4) not accounting for body fragmentation during advanced decomposition
Encasing the body entirely in concrete or within a car will keep the body from coming up but adds considerably to the weight and difficulty of disposal. It also increases the chances of leaving some kind of evidence behind.
