Once you’ve signed in to your WorldTides account you have access to your settings (https://www.worldtides.info/settings) which will show your API Key. The API Key is in this format. At least, mine is.
12345678-1234-1234-1234-1234567890XX.
MMM-SimpleTides is sort of deprecated, in that it has been incorporated into MMM-SORT. However, it should still be working. Now, MMM-SORT does everything SimpleTides does. It has better options for the user. I just fired it up and it is working as expected.
EDIT
I just ran MMM-SimpleTides. It also fires right up
{ disabled: false, module: "MMM-SimpleTides", position: "bottom_left", config: { apiKey: "12345678-1234-1234-1234-1234567890XX", // free from https://www.worldtides.info/developer lat: "40.111111", // your latitude lon: "-74.111111", // your longitude height: "ft", // ft = feet, m = meters useHeader: false, // False if you don't want a header header: "", // Change in config file. useHeader must be true maxWidth: "300px", animationSpeed: 3000, // fade speed rotateInterval: 15 * 1000, // seconds between tides displayed } },