Mariner's Armor

From Dungeon Master's Guide, page 181. Found on Magic Item Table B.

Description
Armor (light, medium, or heavy), uncommon.

While wearing this armor, you have a swimming speed equal to your walking speed. In addition, whenever you start your turn underwater with 0 hit points, the armor causes you to rise 60 feet toward the surface. The armor is decorated with fish and shell motifs.