I am not a doctor so please consult with your own doctor if you actually experience a bout of severe diarrhea, but to answer your question, no, Immodium won't "help" diarrhea, unless you're simply referring to stopping the need to run to the bathroom every ten minutes. Medicines like Immodium just slow down/immobilize your intestines so it's not getting "pushed out" but you're still losing fluid into your gut. After using Immodium, you could become dehydrated without ever needing to sit on the can. Certain infections will also resolve more quickly if you let your body naturally flush them out of your gastrointestinal system.

Better to try and keep hydrated. There are recipes for homemade oral rehydration solutions (ORS) you can make with stuff like table salt, sugar, baking soda (I know I've posted at least two different recipes in the past) or else you can buy a number of commercial products. Pedialyte is good for infants and young children. There are also companies that sell powder packets with the World Health Organization's official ORS recipe. I like and highly recommend a drink powder like Hydralyte (formerly Gookinaid) for routine drinking as well as for more urgent rehydration needs. Ceralyte is another ORS, rice based, which comes in various formulations.