There are multiple extensions of water in Mexico and Central America, some of which are the Gulf of Mexico, Gulf of Honduras, Gulf of Fonseca, Gulf of Panama...
The extensions of water, simply, are bodies of water that are connected with a bigger body of water, but they are going more inland, and are surrounded on two or three sides by land.
The gulfs are prime example for extensions of water, with the Gulf of Honduras being an extension of the Caribbean Sea, while the Gulf of Fonseca being an extension of the Pacific Ocean for example.