Zathabar's advice to have a stamina pot in your hotbar whenever you're at sea is spot on. Always having a stam pot handy when doing anything that includes large bodies of water is a lifesaver for sure. The odd Leviathan have very slippery backs, and trying to stay in one spot while swinging a pickaxe quickly drains one's stamina. I have had quite a few close calls while harvesting chitin through neglecting to manage my stamina wisely. Logging off quickly when stamina is gone while asea, is a cheezy way to get back a full stamina bar. Everyone loves a little cheeze now and then.

keep the ladder of your boat handy and you should be fine even if you take damage from stam loss. Boat placement and orientation are as key as anything since your stam will go down from mining.
They look very like islands, with rocks but no grass and I think if they have a tree at all it will be a small one. One key is that if you get close the minimap won't mark it as land. They are semi rare.

Worth noting, if you are leviathan hunting but still avoiding serpents until you actually get that harpoon, explore ocean biomes during sunny days. Serpents spawn at night and/or during storms, so sunny days are perfectly fine (note that night spawns disappear at day, but daytime storm spawns will stick around their spawn area indefinitely until murdered)...and honestly it's not always great to try and moor your ship next to a leviathan in a storm, especially at night. It does have collision and waves are already known to beat up your boat a bit, especially when the goal is to park as close as possible to it.

Chitin is obtained by mining Abyssal barnacles found on Leviathans which appear as living, moving islands in the Ocean. A 'random' encounter, once located will return to the same position on the map... It appears as a very 'round' island with the large barnacles scattered about on the surface. After a short time 'harvesting' with a pickaxe, the 'island' will shudder and begin to submerge, beware on how long you spend mining the resource.

Good luck. I've come across 2 now and know where to return, ;).

thank you

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

Always have a sta potion on your Hotbar when sailing and going after Chitin! As Compliantdesk said above: you outta sta in middle of ocean? You die!