Chosen answer:While it's never explained, there are several possibilities: as all the tripods were buried deep underground, this one may have been lodged below the lake and was just emerging. Also, a tripod could have "walked" from somewhere else, then went underwater to explore, and, attracted by the ferry's noise and movement, surfaced to investigate. It's also conceivable that, observing how ferries worked, it slipped underwater from further away and ambushed the boat, or the tripod was simply making a crossing from one shore to the other (traveling underwater if it was deep enough) and just happened upon the ferry when it did.
Chosen answer: While it's never explained, there are several possibilities: as all the tripods were buried deep underground, this one may have been lodged below the lake and was just emerging. Also, a tripod could have "walked" from somewhere else, then went underwater to explore, and, attracted by the ferry's noise and movement, surfaced to investigate. It's also conceivable that, observing how ferries worked, it slipped underwater from further away and ambushed the boat, or the tripod was simply making a crossing from one shore to the other (traveling underwater if it was deep enough) and just happened upon the ferry when it did.
raywest ★