I'm not the expert that Ric, Marty and the others are but I'll attempt to paraphrase what Ric has said the last time these questions came up.
The Norwich City caught fire and burned shortly after running aground on the reef. It's fuel tanks were ruptured when it ran aground and the rising water level eventually brought the fuel oil up to the level of the boilers which ignited an inferno. No doubt that it would have been explored by castaways, although it doesn't seem as though there would be much that could be retrieved from it other than pieces of the metal from the ship itself. It doesn't seem as though it would have provided much shelter, but I suppose it's possible.
From the videos that I've seen, the reef flat looks to be very hard rock (edit: probably hard packed coral calcification). It's not sandy so the problem wouldn't be sinking in but rather catching a wheel on the rocks and holes.
The shallow lagoon has been extensively searched with high tech equipment in previous Niku expeditions.
The first satellite photos of Nikumaroro were extensively examined when they first became available. There was speculation at that time of possible aircraft debris just north of the Norwich City, but subsequent expeditions searched that area and came up empty. The 7 site is fairly prominent in those photos.
I wouldn't be surprised if the Naia had a good fish finder. I've heard that the food on board was excellent