Red Chest (The Witcher 3: Wild Hunt)

In the picture is a tag of Geralt under water. Opposite the entrance to the portal to Moreau. There's a darkness of chests under the water and one of them glows red and requires a key. Everything was melted around, all the corpses were cleaned. I remembered that the last time I passed this place, I didn't find the key. I want to understand what's what) thank you!