Escape the river god
Story-related; cannot be missed.
At the end of the underwater area you'll come to an area where a big fish is swimming above you and shining a light down trying to find you. This part can be quite a pain, because you'll think you're hidden but if a sliver of light can go between branches, he'll eat you. You'll have to use your mask to rotate branches to block his view of you. Some parts require you to be patient, while other parts require you to keep moving. Thankfully it's a relatively short section, but dying means you have to start from the beginning of the section. Keep at it and you'll eventually get past him. As soon as he gives up and swims away the achievement will unlock.
Refer to the "Sunset" achievement solution for a video walkthrough of the whole game.