If he put Sierra in water, she would have surfaced by now, unless he weighed her down with something. If they have already done all those sonar searches, and they went throught everything strange they found in the water at both Uvas and Anderson Lakes, then she's not there. Now, maybe they need to begin searching in San Martin, where that old farmhouse is, and the surrounding areas. He just may not have put her in water, and told his mother about Uvas because he knew that she would tell that, he knows her better than we do.
Gases that are lighter than the water build up in a dead body after several days, causing it to eventually float.
