Maybe the car had global positioning technology. Would that work underwater?
And maybe he wanted to return the computer because he already purged the information that he wanted to remain a secret, and he figured that a sanitized computer would be less suspicious than no computer at all?
What was in the PC and wonder what's the state of the HardDrive now? Why'd he bring them back? He could've just taken off. Unless he was just having fun. Back to how did he know what had happened.