So, I am setting up a world for a DND campaign in which, through a magical accident in the past, the face of the globe is covered in ocean, all except for a few tiny islands where the highest mountain peaks were (I am hand waving the obvious problems with altitude and climate as the nature of the accident was essentially wishing for "more water on the planet" and it isn't really relevant to the story). So society has finally recovered from the initial shock and settled into a state of equilibrium. One of the factions that has risen up in the interim is a builder faction who take dirt from the sea floor and carry it to add on the islands or shallow areas to make new landmasses and the party is tasked with protecting one of their expeditions. I was just wondering how to mesh the dredging aspect in believable as it would have to be mostly mechanical in nature, as spell-casters can't function underwater naturally. I have tried looking into how dredging was done it ancient times but there is not much information. Originally I had planned on having each expedition have divers who have strength enhancing and water breathing spells swim down and carry large amounts of dirt in bins but I was wondering if there were more elegant solutions.
So, what are the most logical ways for PC's and NPC's in a DND setting to get old things off the ocean floor?