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.