those are two dredging ships?? I must say I don't understand why couldn't build an ebbankment holding the ocean, then dig a huge hole in the ground. then when all is dug out, gradually let in the water and demolish the embankment. sounds easier than dredging the sea floor.
You would need to build a watertight cut-off wall surrounding your site which extends all the way underground, since marine deposit is generally permeable and water can seep under an embankment and up through the excavation. This is doable but not always easy as concrete may crack due to various workmanship reasons.
Then you need to excavate by stages with an elaborate excavation and lateral support design, and the excavated materials have to be transported away. The ELS will often pose constraints to your plant access which makes it more difficult and hence slower to excavate by large machinery. Also it is way easier to mug out by barge than by land transport due to the excavation volume and logistics involved.
Afterwards you need to demolish the temporary cut-off wall which is not only difficult to remove but also a massive waste in construction materials.
All in all, it's much simpler, faster, cheaper and more sustainable just to do it by dredgers.