Witnesses to the event would be my guess. Plus, to be thorough. The possibility does exist that this was some type of attack. The barge could be safe, yet the pleasure craft infected.
But if you're a terrorist, why spray along a river with two boats when you could hit a stadium, farmers market, county fair, etc.?
Practice run ? Also, how do you know he only hit two boats ? Maybe others didn't notice.
If they were looking for potential witnesses than why not put out search for anyone that could have been in the area instead of limiting it to one pleasure boat ?