Zero degrees latitude is the line designating the Equator and divides the Earth into two equal hemispheres (north and south). Zero degrees longitude is an imaginary line known as the Prime Meridian. Therefore, we are looking for what exists at the location where the Equator and the Prime Meridian cross each other.
If you look at the intersection of 0 degrees latitude (known as the Equator) and 0 degrees longitude (known as the Prime Meridian) on a map, you will see that the confluence falls in the middle of the Atlantic Ocean, in the Gulf of Guinea off the coast of western Africa.
Therefore, the open ocean lies at zero degrees latitude and zero degrees longitude, marked by a weather observation buoy, part of the Prediction and Research Moored Array in the Atlantic, also known as PIRATA.
There is a fictional island known as null island at zero latitude and zero longitude that can be found in the public domain Natural Earth GIS data. This one square meter area island is used for troubleshooting purposes for flagging geocoding failures.
Map of Zero Degrees Latitude and Zero Degrees Longitude
The map below shows in dashed red the lines of zero degrees latitude and zero degrees longitude.