I have a civilization that lives not exactly underwater, but completely surrounded by water in which their cities are both submerged underwater or built floating right on top of it. Almost all of their resources are limited to the ocean. What materials would be used for them to make clothes out of? I have ideas such as shark/whale/seal leathers. Would seaweed be possible? Or what other oceanic resources would they use. Or even for building resources and shoemaking, etc.
Fish leather is a remarkably soft clothing material and is a very low technology option. You can find simple recipes on YouTube that require little more than a trout skin, some vegetable oil and eggs. If your people are surrounded by water and have access to plant based oils and eggs (seabirds perhaps) than a soft textile is well within reach. For hardier clothing marine mammal skins would be better and are made with Stone Age technology as evidenced by the Inuit who live on ice sheets.
Shell bikinis are great but they have been so done. Shell codpieces really not much! Nix here looks good in his and your characters will look great sporting their fancy man-shells.
You can do lots of other things with shells - string them on strings to make stuff, glue them on - shells!
Byssus. Difficult to put together enough for clothes, but people could gather some byssus every time they harvest mussels for food. The clothes would be expensive, but it is doable.
Fibre from sea weed? I don't know, In water there is not evolutionary pressure to grow fibres strong enough to sustain their weight, but on the other hand algae growing close to the shore must be able to resist the rough shaking coming from the waves, so there could be sea weed with strong enough fibres there. But I suspect they would be used only for the cheaper clothes. Actually I just realised that probably you can get fibres strong enough to make clothes from the root of sea weeds. To keep them anchored to the sea bottom they have to be strong.
Apparently whale baleen can be used to make baskets https://en.wikipedia.org/wiki/Baleen_basketry
Perch skin used to be treated and used in Sweden as a cheap alternatives to other skin types, when smaller amounts of skin were needed. Perch skin is pretty tough and since perch are plentiful, it would be possible to use it for clothes, although using hides from larger animals is a lot easier. But were there no larger animals available, one would have to do with what is at hand.
Searching on Google for perch skin, I found this article from a Swedish university, where skin from salmon, cod, perch and lumpfish were prepared and tested in regards to strength. The writer claims that the skin types are strong and long lasting enough to be used for furniture in public settings. This requires high strength leather, so clothes should not be a problem: http://www.diva-portal.org/smash/get/diva2:331808/FULLTEXT01.pdf