Well, the correct way to write it is in two words 물 + 아니에요. So, if you were to say it slowly, the pronunciation is "mul ani-yeyo" (this isn't the normal way to romanize it, I just wrote it that way to help you phonetically). However, in any language native speakers speak at a relatively fast rate and so words string together. So with that being said, when you say it fast, it will sound more like "murani-yeyo". Since the last consonant/받침 is 'ㄹ' and the first character of the next word is a vowel, the ㄹ sounds like an 'r', resulting in "murani-yeyo". If you're wondering about the pronunciation of 아니에요, it would be romanized as "ani-eyo/anieyo", but to make pronouncing it easier a 'y' sound is made in front of "eyo/에요", so it sounds more like "aniyeyo"

이거 물이에요. igo muriejo 물 아니에요. mul aniejo

I am struggling a lot with pronunciation (I'm hearing impaired and can't hear some sounds). I will be watching your question for an answer. A couple more pieces of information that the answerer may choose to refer to: R.R.: igeo mulieyo mul anieyo My version of how I think it is pronounced based on what I know at the moment (without accommodation for the jamo sounds that don't exist in English): ee-guh moo-lee-eh-yo mool ah-nee-eh-yo