"This pond's water is cold."
Translation:इस तालाब का पानी ठंडा है।
No, the postposition should agree with the gender of the object ie पानी not तालाब as तालाब is a subject.
No. The postposition का always takes the gender of the word that follows it (i.e, the thing being possessed by the object of का)
So, it is तालाब का पानी (water of the pond - because पानी is masculine) but तालाब की मछली (fish of the pond - because मछली is feminine)