The term 'What 'La Mer' is made of' can be answered with the three letter word 'EAU', which is French for water. La Mer is a brand of luxury skincare products, and its signature product is a moisturizing cream made with sea kelp and other natural ingredients. The cream is said to be made with 'miracle broth, ' which is a combination of sea water and other minerals. Therefore, the answer to the crossword clue is 'EAU', as La Mer is made of water.