Someone started it by asking sarcastically if an unidentified killer waited until he knew that Scott had gone fishing in the bay and that it was being searched then went and dumped it in EXACTLY the right place.
Someone else (howlin I believe) said that that hypothetical person would have had to know EXACTLY where to dispose of the body.
My point has been almost 100% the same as yours, no one could know EXACTLY where the body would emerge.
I also contended that if I, not Scott had murdered Laci, and had later learned of the search in the bay, I would have retrieved the body and dumped it in the pay.
I would have done this to insure that the trail led away from me, too bad about the implication of Scott.
And another coincidence -- they found Scott Peterson's computer had been frequenting a site where he was studying the currents in San Francisco Bay. Fry Scott Peterson!