Hi Iona,
I know how hard this is and it is way too early to expect your pouch to have adapted yet but there are things that can help along with Scott's answer.
What is your diet like? What is your eating pattern like?
Is it 100% watery all of the time of sporadically only?
Your biology is trying to get used to its new plumbing and its functions so you will require around 6-12 months of adaptation but this dehydration is very worrying.
Do you eat bananas? Peanut butter? Try boiled and freshly mashed potatoes with olive oil and sea salt...it is my go-to recipe for liquid output (when I do not want it to be liquidy...K pouch here)...
Certain foods will help and others will complicate the problem...I know that the Brat diet suggests rice and white bread toast but you may wish to try whole-wheat toast or English muffins, it seems to absorb the excess liquid better (at least in my case)...I never use margarine but only pure butter (vitamin A).
Charcoal is an old-fashioned recipe to combat severe diarrhea...you can buy it in tablet form at the pharmacy or just 'burn some toast' (my dad's recipe when I was a kid...I developed a taste for it, since then!).
If you can find it (and this may sound yucky) but get plain, flavorless jello or lime-flavored and make it into a salty snack using lemon, salt, mint (mojito-flavored????) and use only 1/2 of the liquid to make it or find other flavors that can go with salty flavors...it makes it easier to keep up the sodium and the potassium if you use the right ingredients...and keep it handy.
Have you tried eating dates? Majoule Dates are big and meaty dates and for some reason, they seem to thicken-up output really well, a full of minerals and taste yummy...and make a good dessert.
I hope that your output slows down...
Sharon