Claymarine

The Claymarine is a Hardmode sword exclusive in Expert Mode. It is obtained by fishing in Hardmode, where it has a 1 in 100000 chance to be fished from water and a 1 in 50000 chance to be fished from lava. The catch chance is increased by fishing skill.

Its best modifier is Legendary.

Enhancement
The sword can be upgraded throughout the game by talking to the Pandolar Salvager with the item selected. Clicking the Claymarine Enhancement button (which replaces the salvage button) while holding the sword opens up a UI with two slots: One for the sword and another one for the Mystic Stone. The weapon starts out at 45 base damage and 4 crit with no additional effects. All enhancement stages are shown in the tooltip of the weapon.

Trivia

 * This sword is inspired by how the Calamity Mod's Legendary Weapons used to work, the only difference being the stat increase working differently and that it is obtained by fishing rather than being a monster drop.
 * However after 1.1, this no longer applies as the system was completely overhauled.
 * As of now, the Claymarine's max damage at the end of the game is 550.