Duggap is correct, it is an imitation of an actual US Navy button, manufactured for use on civilian coats. (Button-collectors call civilian-usage buttons with an ornate emblem "Fashion" buttons because they are made for the Fashion clothing industry.)
As Duggap indicated, the main ID clue that your navy button is a civilian-usage imitation is that it has an iron back. (Sometimes the iron back will be rusted out.) Other ID-clues that your navy button is a non-military imitation are:
1- The rope encircling the emblem (called a "rope border") does not pass through the ring on the anchor's top.
2- Very "rounded-top" wings.
3- Low-quality detailing in the die used to stamp the emblem into the metal button's front.
Compare your button with an actual civil war era US Navy button's deeply-detailed emblem, pointed-top wings, and anchor-ring position, shown in the photo below.
This button was found at a local park in central Indiana. The park was established on the site of an old farm which I believe was built at the beginning of the 1900's. I was detecting over where I believe the old home once stood. I did not find any coins to date the area, only some old rusted clock parts and a harmonica reed. I found a short square nail that I suspect was a horseshoe nail. The design of the button is obviously that of a Civil War naval button, but I do not know if this is a legitimate Civil War button or a later reproduction. Here is a picture of the back. 
