Here is a faction of one of my worlds, the Dkari. An ocean-based society. I want to know how feasible some of this is.

"Their ships are built mostly from the bones and hides of aquatic mammals, with some wood salvaged from other ships. They have small amounts of metal, either likewise salvaged or traded for. The bulk of their diet is likewise seafood, supplemented with trade. They rarely set foot on land- indeed, many of them are lithophobic- and spend most of their lives aboard the ship they were born on, unless they chose to move to another ship upon coming of age or marrying. Abandoning the sea entirely for land life is exceedingly rare, and those who do so are considered to be traitorous exiles."

"They live across a vast stretch of the southern oceans, where they have a near-monopolly on sea-travel. There are many populated islands in these seas, but the Dkari do not live on any of them. They are home to other, smaller, partially self-contained societies. The Dkari largely leave these enclave-islands to their own devices, but forbid them from building or sailing ships beyond a certain distance. Trade and migration occur only through the Dkari or with their permission, and they charge a heavy price for both."

I have three main concerns for this:

Diet. Is there any way to have a lone ship or small fleet me mostly self-sustaining with minimal land contact? I know that certain real world cultures- the circumpolar peoples- the Inupait of northern Alaska and parts of canada being a subset- come to mind, i'm sure there are others- live on diets of mostly fish and seals, but what about scurvy? And aren't pregnant people supposed to avoid fish?

Shipbuilding. How feasable would decently large ships of mostly bone-and-leather be? I mean something on the scale, though presumably very different design, of those during the Age of Exploration. And disecting and processing most of this material while at sea?

Geopolitics. If the above two problems can be solved, how feasible is it that this society could become a dominant regional superpower? Especially along the specific lines detailed above. The broad concept, called a Thalassocracy, has existed in the real world, the Delian League and certain eras of Austronesian history being examples, but I could find none with this particular or extreme of a relationship. Could it be made to work?

Edit: formatting and spelling.