A couple of thoughts...
They could have just been out for a boat ride, or, they were taking photos.
Even though there were no photos of the bridge on the cell phone they could have dropped the real camera into the water to dispose of the suspicious evidence. Or, the brother that went back to his car and later turned himself in to the police could have stashed the camera somewhere for later retrieval. If he was smart he wouldn't have stashed in in his car. Hopefully, if there was a camera, he isn't that bright.
Camera-wise, some of those cheapie $4.99-$9.99 disposable cameras are good for use in water. We took underwater photos in the Caribbean with them rather than spend $$$ on a waterproof camera. These cameras float, therefore they could go back and retrieve the camera later...