This achievement is for not scaring the Lobster Girl throughout the game.
This is really the only missable achievement since there is a save system.
If done successfully it will unlock at the end if Chapter 5 when the Lobster Girl returns to town and Unlocks Chapter 6 for you.
There are 4 rooms in the game at various Parts where the room will have a big port hole in the middle showing the underwater vista. In these rooms The Lobster Girl will swim up to the port hole and look in on a constant loop. If you run, walk, or fight in the room you will startle her and she will swim away permanently. You have to crawl through each of these rooms.
Note on A few instances you may need to backtrack through one of these rooms. Be careful to crawl at these times too.
Room 1 - Tour Route
Room 2 - Mossy Biome 1
Room 3 - Laboratory
Room 4 - Mossy Biome 3