I have had terrible diarrhea all night (ate way too much fibre). how can i rehydrate today? replenish electrolytes? is coconut water a good way to go?

Fluids: If the diarrhea is gone you can pretty much drink anything and your fluid losses will be replenished. The body is smart and separates the solutes (sodium, sugar, etc) from the water and conserves the water and utilizes the solutes as needed. Coconut water may actually induce diarrhea and i would avoid it in large quantities. If you still have diarrhea try pedialyte or gatorade. See md if fever.