The sea of regret on the minimap (left). Note it has no map icon.
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.
Using Flasks repeatedly can allow the player to survive in the Sea of Regret for short periods or time. It is also possible to use a pair of Wings or certain Mounts to fly over the Sea of Regret to avoid damage.
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".
The notification you get when trying to fish in this biome
Sea of Regret (Right) and Sea of Tranquility (Left)