I dehydrated many times when I had my loop ileostomy; to such an extend, on each occassion I was rushed to hospital by ambulance for IV rehydration.
I was advised that, as a guide, I should consume enough fluids so I urinate at least 3 to 4 times per day, urines hould be of a light straw colour regardless.
If feeling fatigued and suffering from cramp, then your electrolytes require replenishing.
When I feel fatigued, I drink the St Marks emix solution.