I've been told mixed things about hydration with an ileostomy. A lot of websites say you need to drink extra, because you're no longer absorbing the fluid back into your body that would previously have been absorbed by your colon. However, I've also heard that drinking too much will wash things through your system too quickly and actually make you more likely to become dehydrated. My own surgeon told me not to drink too much, as I'd not only wash all the liquid through, but food as well, making it harder for me to gain weight (I'm very underweight).
I generally listen to my own surgeon, but he comes out with odd things at times and I've not yet worked out if this is one of them. My second surgeon told me there are no set guidelines - if you have symptoms of dehydration, you should look at your fluid intake and experiment with more or less, different foods, etc. to find what works. This is presumably only if you're mildly dehydrated - if it's a serious thing, I think you're best checking with a stoma nurse or doctor in case you need IV fluids.
I've also been told, I believe correctly, that you should increase your salt intake too, as the colon would previously have been absorbing salt, the same as it was absorbing fluid. But many people have more than enough salt in their diet beforehand, so again only address this if you're getting symptoms.
Personally I prefer watery output. I know it's not supposed to be the healthiest, and we're told to aim for "toothpaste" consistency, but watery output doesn't leave such a mess at the end of the pouch when you empty it. When I have toothpaste consistency, I change the bag very often because it never seems clean. I usually do have toothpaste consistency, I get watery output maybe a quarter of the time, but it seems unrelated to what I've been eating. But then, before I had a stoma I would have bad diarrhoea days regardless of diet.
There are many foods, many already mentioned, that can slow and thicken output - foods with pectin in (jelly babies, apple sauce), smooth peanut butter, white bread and white rice, cheese, bananas. But for me these make no difference. I often do stick to low fibre foods, but to avoid blockages, which are a big problem for me, rather than to slow things down.
Medications - codeine, loperamide, co-phenotrope can all help slow things down too. My reactions to these medications have been surprising: before I had an ileostomy, loperamide and co-phenotrope really helped with diarrhoea if I took high doses; codeine seemed to have no effect. Amitriptyline (which wouldn't ordinarily be prescribed primarily to treat diarrhoea, but which causes constipation as a side-effect) had an extremely constipating effect on me when I had no stoma. But with the ileostomy, I can take all these meds with no difference whatsoever. That doesn't bother me, as I said, I find watery output preferable, but I find it odd. We seem to have very unpredictable reactions to medications, so it may be worth checking with your doctor if trying any of these or upping the doses can help you.
Have you been measuring your fluid intake as well as output? When I've been in hospital for various things, doctors have tracked both - that is, urine and stoma output are measured and compared to the amount of liquid I'm drinking. This can give you a good idea of what's going on, though I'd imagine it's a bit tricky to do this when not in hospital. Despite have watery output fairly regularly, my total stoma output volume was actually on the very low side of normal. I've had three surgeries in which a new ileostomy was created, each time my output slowed down within a few days of the surgery.
I wouldn't worry too much unless you're getting symptoms of dehydration (check to see if you have a low blood pressure, check the colour of your urine, and pay attention to how thirsty you're feeling). Hopefully some diet changes and maybe medications, or products like gel you can put in the pouch to absorb liquid will help make the output easier to manage.
