What you are looking at is a long leg house jack with a cradle on top instead of a plate. If you come across one of these let me know, we have been looking for quite some time. I know of some older ones, but last I checked they were still not for sale.
You can do the same thing with a hydraulic bottle jack and a 4x4 cut to the remaining length. Use the jack above water with the post going into the soil below it, that way you do not need to go underwater to release the pressure. You could put the jack underwater as they are sealed units, just wipe it down when you are done. Just remember when jacking any heavy object, keep things as plumb as possible to keep from shifting things around.
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