Building a subsurface that will capture fine gold is a tough job and requires the understanding that pressure drives the flow of water/material unlike a surface unit which benefits from gravity flow and angle to help move material over the sluice. This type of system has been deployed on surface designs also but does require considerably more design than a standard subsurface dredge.

This type of system will achieve much better recovery than a standard submersible because the flow rate to move the smaller material is much less than the requires flow to move the large rocks. The sump is then run as a separate system with it's own suction jet drawing from the sump and feeding the sluicebox. I believe material is brought to a screen separator which allows the larger material to be discarded and the smaller material is caught below the screen in a sump. It was hard to see much of the 'Dragon' on the show to give a good description, but from what I saw I think he is running a 2 stage system.

Small dredge set up