Just off the coastline of the odd continent float large passive creatures called Leviathan. They are large tortoise like creatures with giant barnacles on their shells. Mine these barnacles for chitin, but beware, while mining the chitin Leviathan will become active and submerge, so have your boat moored alongside them. I always begin mining the barnacles on the side opposite my sailing vessel then work across its back toward my vessel, being cautious not to let my stamina get too low in case I have to swim.

https://valheim.fandom.com/wiki/Leviathan