Here's why:
* Proper nouns are specific names of people, places, or things. They are capitalized. Examples: John, London, Amazon River.
* Common nouns are general names for people, places, or things. They are not capitalized. Examples: man, city, river.
"Water moccasin" is the common name for a specific type of snake. It's not a unique name, unlike the name of a specific person or place.
Therefore, it should be written as water moccasin, not Water Moccasin.