Delmarva

In the world of Steven Universe, "Delmarva" is a state on the east coast of the United States where Beach City is located.

In Steven Universe
Ronaldo Fryman states on his blog Keep Beach City Weird that Old Bay is popular in Beach City, implying Beach City is in the general area of Maryland. In the same post, he states that they have to put Old Bay on everything as they live on Delmarva, providing further evidence for the claim. Additionally, Ronaldo also mentions that DelmarvaCon is held every year in Charm City (a nickname for the real-world Baltimore, Maryland) which "isn't too far of a drive" from Beach City.

According to Mayor Dewey, as well as a comment by Fryman, there was once a city in Delmarva that was close to Beach City called "Ocean Town". It is implied that due to an inadequate mayor, riots and other unsolved issues within the town led to the town's complete downfall. The destruction of Ocean Town was apparently so great that Steven didn't even know it existed.

Behind the scenes
In the real world, the Delmarva Peninsula is a large peninsula on the east coast of the United States. Its name comes from Delaware, Maryland, and Virginia combined into one word. The Delmarva Peninsula is a flat and sandy region mostly associated with coastal cities and seafood; it is the location for the fictional town of Beach City in Steven Universe.

Trivia

 * Due to Steven Universe being set in an alternate earth which has been influenced by gems, Rebecca Sugar stated that Delmarva is its own state within the show.
 * This could explain why Beach City has a unique state flag, rather than one of the three states that encompass the real-life Delmarva.
 * The Delmarva flag was implied by Rebecca Sugar to be that of the snake found on both Beach City's money and also on educational messages seen in certain arcade games found in Funland Arcade.
 * Since Delmarva is a state within the show, it can be assumed much of the Maryland, Delaware, and Virginia areas don't exist as unique states.