@georgechalhoub Another more likely possibility just occurred to me: that the report means that all the watercraft operating in California together produce in 2 hours the same pollution as one car driven 100,000 km. Then Stienstra's mistake could be to interpret that as meaning that one jetski operated for 2 hours produces that amount of pollution. I think this is a better fit both with the logic of the situation and the language of the report.

Here's a question relating to this issue from A Tale of Two Cities

Is something missing from the start of this answer? It appears to be the tail end of a sentence.