Sponge

According to Encyclopedia Spongebobia, the dollar bill in the Spongebob universe is the same currency as the U.S. dollar. the above picture does not correspond to any real U.S. currency, but based solely on the fact that the pictured fish has a top hat, it seems to be a fictionalized Abraham Lincoln (Abraham Lingcod) (I made that up)

This image of suspected Lincoln, however, doesn't match or even look like the version of him on the Bikini Bottom penny.

The creation of money in the Bikini Bottom is similar to real life currency minting, with a lot of cartoony wacky additions, such as "a step where old money is burned up and created into newer money". The mint is stated to be based off of the United States Mint in Philadelphia, USA. In S8 E161b, "Mr. Krabs takes a Vacation", the episode of Spongebob that features the mint, two unique dollar bills are created with Spongebob's and Mr. Krabs's face on them.

The official statement made on the currency of the Bikini Bottom is:

wait, the Bikini Bottom is a part of the United States? they have a king, a national anthem, and a president. It's even shown in the opening shot of the theme song, as well as referenced several times in many episodes, that the Bikini Bottom is located below Bikini Atoll, in the Marshall Islands.

But enough of that.

Let's assume, then, that the Bikini Bottom is part of a larger underwater nation, rather than a standalone city. After all, the currency is widely accepted even outside of the metro area of Bikini Bottom. Let us further assume that, while emulating the United States dollar, it is its own separate currency. This still tells us close to nothing.