Answer: Gantry cranes with submersible dry dock. It has to be a crane because we have to accurately place one hull on another.
Vessel is Blue Marlin. Load has been described as 22 barges. There are barges there to allow hulls to be transported, but mainly inland ship hulls with super structures on top of hulls.
Destination was Veka shipyard Lemmer, which produce inland vessels. Largest of these have a tonnage of 6,228 tons. Let's say 5,000 tons (this is a way over estimate, but good enough to answer question).
Super-structures (that it is) can be clearly seen on top.
Sea trials of one or a similar hull.
To lift 5,000 tons onto a barge, we need gantry cranes with water access.
30,000 ton gantry crane with submersible water quay in Singapore by Sembcorp Marine. The legs are on wheels so the gantry cranes can come closer to each other to lift loads (Second image).
YouTube Video: The 15,000 tons record transport project on Cometto SPMT Self-Propelled Modular Transporter of cranes ariving in Singapore.