The fact that every Radio, TV station and newspaper in the entire state was reporting that the police were searching for his dead wife in that location isn't a "plausible reason" for him to be there?
Guilty or not, if they were searching in a very public way for my dead wife I would go out and watch.
I get the impression that Scott did not go out to the Marina to watch the search in a public way. Sounds like he was lurking and thought that no one knew he was there. Keeping tabs on how close the police might be to where he dumped the bodies.
If he had nothing to hide, why bother sneaking out there. He would know Laci and Conner were not there, and would focus his efforts elsewhere.
Reminds me of Robert Chambers, lurking with the bystanders when they found Jennifer Levin in Central Park.