They are probably all equally common and equally correct. Sometimes either would be okay, and sometimes the context would make or or the other more likely. If we were trying to find your particular cat in a house where there are many cats, I would probably say "there are SOME cats in the kitchen; maybe one of them is yours." But if I was in charge of the kitchen and there was a rule that cats were not allowed in the kitchen, I would probably yell, "There are cats in the kitchen! That's against the rules! Get them out of here!" With your water example, I can't think of contexts that really make either one more likely. They both mean the same thing and are equally likely.

Adding the adverb 'some' to the noun 'water' or 'cats' shows a difference in quantity. Most people would think that some is less then not using some. Other adverbs that could be used (to show difference in quantity) would be: lots of, a few, a clouder of, loads of, thousands of, ... I would say they are used more in written English, both sentences. For spoken if someone saw cats in the kitchen they would probably ask a question rather than make a statement of fact. Why do you have so many cats in your kitchen? Watch out for the water on the floor. Watch out you have some water on the floor. But again adding some could come up as many times and as Nathan suggested above they are equally common. Then of course if there is only one cat it would be: There is a cat in the kitchen.