Strategy guide

What to Look For

Abyssal Barnacles: Found on the backs of Leviathans (floating islands that look like rocks). Mine these with a pickaxe to get Chitin. The Leviathan will submerge shortly after you start mining, so start with barnacles further away from your ship and then try to get the ones close to your ship when the rumble starts. Remember that you'll take damage if you try to swim with no stamina left from mining.
Serpent Meat: Dropped by Sea Serpents. This is the ingredient for Serpent Stew, one of the best health foods in early-mid game.
Serpent Scales: Dropped by Serpents, but they sink in water. To collect them, you must use an Abyssal Harpoon to drag the Serpent onto land before killing it.

The Harpoon Strategy: The Abyssal Harpoon (crafted from Chitin) is the most important tool here. While you can kill Serpents with a bow, harpooning them allows you to control their movement and drag them to shore for their scales and trophy.
Weaknesses: Serpents are weak to Frost damage. Using Frost Arrows will slow them down, making it much easier to keep your ship from being destroyed during an encounter.
Ship HP Management: Always carry materials for a Workbench and a Hammer on your ship. If a Serpent damages your hull, sail to the nearest coastline, hop out, drop a bench, and repair immediately.

Never explore the deep ocean without checking the wind. If you are sailing into the wind, you are a sitting duck for Serpents. Use the Moder Power when you absolutely need wind always at your back, like during a night storm when risking an attach from a sea serpent.
Sea Serpents spawn significantly more often at night and during storms. If you are low on health or your ship is damaged, stick to the coastlines until morning.
The Longship can outrun sea serpents if you have the wind in your favor.
The Longship is the only viable vessel for serious Ocean hauls. It has 18 storage slots and significantly more HP than the Karve, allowing it to survive multiple Serpent bites.