The only report I got so far from the diving group was from someone who has been diving in the area many times and hasn't seen anything. He pointed out that things don't last long under water, but these locomotives were likely heaver materials and sunk after the
wheel sets at Manukona, which are in pretty decent shape for so long under water.
There's also supposed to be a
submerged narrow gauge locomotive in a Kauai bay, but I don't have any connections to local divers there and to dive it I'd need a buddy to go along, an airline ticket and at least one overnight on Kauai, so while it's on my list to check out some day, it won't be for a while. That bay has a small harbor, so it's likely that people have been all over it. The similar harbors here on the Big Island have periodic clean-up days where people scour the bottoms for debris. Visibility is usually really bad in the harbors, but it still seems likely that someone would have found it if it was there.