He's a narcissist. He didn't think he'd get caught, since he weighted his wife's body down to keep it in the water.
He didn't know that currents would tear the body apart and the torso would float to the top.
Scott probably thought that people would never suspect he actually disposed of the body at the same area he said he was fishing. OR he could go on to plan B and say he was "framed"..
Looks like he found someone who didn't disappoint him :~)
sw
He was in a densely populated area and any number of people could have seen him (and perhaps did and have submitted statements to the police detectives). He could have been cocksure that the body would not surface because he wrapped it in a tarp, tied concrete blocks to it, and dumped it way out in the bay.
I am guessing because he was worried someone might have seen him at or around the scene and might remember having seen him, and he was covering that potential situation. Also he didn't expect Laci's body to break loose from the concrete he had it tied to. That's what I would guess. All accomplished liars mix fact in with their fiction to make the fiction more convincing.
He had no way to know who may have witnessed him in the area near the water, or on his journey to and from the water. Naturally he had to assume someone saw him. He was out in public, after all. So he constructed an alibi that a) depended on the hope that his wife's weighted body would stay submerged, and b) assumed he had been seen on his "fishing trip."