What is the body of water between what is known as Baja California and Mexico?

It is known as the Gulf of California.
The Gulf of California is a marginal sea of the Pacific Ocean that separates the Baja California Peninsula from the Mexican mainland. It is bordered by the states of Baja California, Baja California Sur, Sonora, and Sinaloa with a coastline of approximately 4,000 km (2,500 mi). Rivers which flow into the Gulf of California include the Colorado, Fuerte, Mayo, Sinaloa, Sonora, and the Yaqui.

The gulf's surface area is about 160,000 km2 (62,000 sq. mi).
The Gulf is thought to be one of the most diverse seas on the planet, and is home to more than 5,000 species of micro-invertebrates. Home to over a million people, Baja California is the second-longest peninsula in the world.