If you are in a ship and don't have the gear required to fight a sea serpent (for example) empty enough of your inventory into the ship's cargo hold (or toss stuff you don't really need over board) and make sure that you have enough weight and room in your inventory to hold all materials needed to construct the boat (and make sure you have auto gather turned on) then move to the mast of the boat and bravely meet your fate. When the sea serpent destroys the boat the items in the cargo hold turn into floating crates and the materials that built the boat get autogathered into your inventory and when the sea serpent kills you your corpse floats so you don't lose the materials. If you don't get back to the crates in time they will despawn (3 ish game days) but corpses last until picked up.