This isn't a hard one to get, but since it's a Secret Achievement you wouldn't likely know it in advance.
As you adventure throughout the island underwater, there are many windows out into the waters around.
There are a few rooms however that primarily are just a mostly empty room with a window in the middle that will have the Lobster Girl curiously looking inside the room.
If you do anything other than Crawl within a certain area of her, you will scare her off and she will never appear in these windows again (thus failing the Achievement for that continued playthrough).
If you DO startle her, just quit and reload your save file and try again. (This can be a hard decision if you have done a lot of things since your last save.)
Once you reach a certain point in the game (close to the end), she will show up with your other friends and you will get this Achievement.