The first types of gold dredges in the past were the underwater sluice type. We still have these types of dredges these days but they are called submersible and they have changed a bit from what they were in the past.
The underwater type or submersible types of dredges the centrifugal pump stays on the surface and water is aimed through a long high pressure hose to an underwater tube that does the job of picking up material to the suction nozzle and moves it through the tube and on top of a small sluice that serves as an important part of the suction tube.
Underwater dredges are also jet pumps and the suction that occurs is usually the same or better than the one of the ejector mounted higher type. Due to the fact that the jet does not need to provide air pressure and gravity since it is working in a level position, all of the energy can be used to push the water in the tube and trap water at the bottom of the jet. Oddly enough, air pressure is not an advantage to the underwater dredge since its effects functions on the intake and discharge side of the suction tube. The flow of the stream does not serve as help to the head of the jet because the jet needs to dispel its extra energy into the stream. A few feet past the discharge exit it slows down to the same speed of the stream. This causes there to be pressure inside the tube that needs to be surmounted.
This is not an inconvenience, but the jet provides more than enough power for the tube and the suction that is caused from it is very powerful. One of the set backs of the underwater jet though is that because of its design the sluice has to be smaller therefore it is not as efficient as it could be. This causes the dredge to move more material over a smaller sluice at higher speeds and can cause a loss of what you are looking for.
Manufacturers have come up with a lot of new innovative dredges though and a lot of the problems that existed in the past are not a concern these days. One of the dredges that were created was the combination dredge for example. The combination dredge is a fixed, close to the surface ejector and a surface sluice type of dredge so to say. When it is used as an accessory it is prepared with a longer high-pressure hose that can be attached to the underwater suction tube.
In the previous design the dredge can be utilized to move a good amount of overburden in a shorter amount of time. After the dredger decides he is close to the are where more gold can be located, all he has to do is change the high pressure hoses and switch over to the surface sluice, very simple.