I have a culture, superficially similar to the Inuit cultures, that lives on numerous small islands far to the north. Many of these islands are devoid of deer or other large mammals and on a number of them, the only source of food is lichens and marine mammals; seals and whales. So my question was, could whale and seal skin be used to make leather? as far as I know the Inuit used reindeer for that, but my culture lacks this resources. If it can't be used for leather, what might replace it? The islands are mostly rock with some topsoil, sparse scatterings of trees on a few and numerous lichens and bird species.
Seal skin has been used for leather for a long time. Indeed, it is leather as all skin (including human) when properly tanned is leather. It's really a question of how thick it is. Human skin isn't thick enough to make a good leather.
Whale leather also exists, but some comments online suggest many species' skin is too thin or rubbery to make what we humans would consider a good leather. Nonetheless, not finding a lot of whale leather products today may have a lot to do with decades of sustained Green Peace effort.
But, just to make a point, for a while you could get your luxury SUV with whale penis leather seats, but apparently that had some complaints and withdrew the option (if it was ever really an option and not just a publicity stunt).
While whale and seal can both be used as leather, I would actually suggest a sealskin coat that is stuffed with some kind of fine lichen or maybe witches hair lichen. Other stuffing could include moss, air bladders from fish, or shredded fine bark.
(Edit: Seal Skin has been used traditionally by natives because of its waterproof qualities.)
Most fish can actually be made into a leather, but skate, rays, halibut, and Salmon all seem like the most prominent or likely choices. Shark is also used for leather, though I don't know how it compares to seal, which is definitely good for your purpose.