The Sea of Regret is a biome that appears around the edges of the map in any world. It has a darker shade of blue than regular water. If someone goes into the Sea of Regret, their health will be drained over time and their health regeneration will be disabled.
The Sea of Regret does not act as normal water and cannot be fished in or collected. If you throw a lure or sponge into the Sea of Regret, it will be returned to you. When attempting to fish, a notification will appear saying, "You have not yet collected a pole that can fish in Regretful Water".