They have a very close meaning, but in some sentences it's to better to use one word than the other. ex: المزارع رش النباتات بالمبيدات الحشرية. The farmer sprayed the plants with insecticide. رذاذ الماء يتساقط على وجهي. The water drizzle falls on my face. (I'm not sure sentence is right or not but I mean drizzle from a waterfall or rain) and in your question رذاذ الماء is right. These two links may help you and give you a better idea about when yo use each word - https://www.almaany.com/ar/dict/ar-ar/رش/ - https://www.almaany.com/ar/dict/ar-ar/رذاذ/

رش فعل اما رذاذ فإسم رش يرش رشاًَ رشني الكورنيش بماءه

رش =when you use a spray or perfume رذاذ= just when you feel something a little wet or just little drops of water

الرذاذ صفة تصف المياه عندما تكون حبيبات المياه صغيره جدا اما الرش هو فعل يتم عمله لتحويل المياه الى رذاذ وهذا مفهوم مبسط