Sovereignty over land defines nation states since 1648. In contrast, sovereign right over the sea was formalised only in 1982. While land borders are well-known, sea borders escape the limelight.
Any littoral state claims the area stretching 200 nautical miles from its coast. This area becomes an Exclusive Economic zone (EEZ). Complex bathymetry of the ocean marries the Law of the Sea to shape quite convoluted maritime borders. You can view an accurate representation of seabed or color the zones and hover over the disputed sectors.
The beauty of complexity is apparent when you explore the areas that cannot be grasped by any neighbouring state. These are the so-called donut holes:
Gulf of Mexico
Sea of Okhotsk