I feel a bit sorry for you op, for the rubbish advice you've received from the usually helpful DameDiazepamTheDramaQueen. You could have sorted this 9 months ago at 9 months old. Much easier then than now, with an 18 month old toddler.
Not giving a weaning baby water? I've never heard anything so rubbish. Your baby needed/needs to quench thirst and is now weaning. Sounds like someone mistook the advice given to a pre-weaned baby baby on water in place of formula.
That could be argued in the same vein with regards to diluting formula. It is not recommended because it stops baby getting the nutrients from the milk. The point being that at 9 months plus, baby did not need the nutrients at night. So watering the milk would have been fine. It's about taking things in context, not just reading Dr Google with no real thought.
And the nutrients in milk being a me to the next two misconceptions on this thread: that night feeds until 12 months are necessary and to be expected (absolutely not) and that baby needs the milk given in the night. It's different for a breastfeeding parent, where night feeds are less about milk and more about comfort. But on formula, absolutely no reason baby couldn't get all of their calorific needs in the daytime.
At 9 months old you needed to ensure a high calorie diet of solids and plenty of milk in the day, then just stop the formula at night.
At 18 months it's going to be harder, but the same thing. You just need to stop the night feed and find a different way to comfort baby that doesn't involve milk. Basically The advice now is what you should have done 9 months ago, but will be harder now not least because baby can stand (and climb) and scream louder when unhappy in the cot.
I would not replace the milk feed with anything else, because that could become the same crutch as a night feed. Just go cold turkey.
Also completely seperate milk and sleep at bedtime and naptime. Don't give a bottle straight before going to sleep. Develop an alternate comforting mechanism for baby - comforter toy, patting, shushing or whatever works. Just no milk. At all.
Oh, and keep your child well hydrated during the day. Water is important for the child. A child well hydrated during the day should not be waking thirsty at night, just like a well fed child should be more than capable of having no calories for 12 hours overnight.