With the exception of far north Baja (like Tijuana), the Pacific is just as warm as the Gulf and Caribbean in Mexico, although Cabo's waters in winter (while still warm) are cooler than Cancun's winter waters, or the winter waters of Veracruz or Mazatlan. This is simply because Cabo is further north.
Veracruz (Gulf of Mexico):
The thing about the Gulf Coast of Mexico is, while there are some crystal-clear areas, most of the beaches look like this. The ocean is warm year-round thanks to the tropical climate, but the beaches aren't any more impressive than the beaches of Mississippi, otherwise.
Compare that to these places below.
Cabo San Lucas (where the Pacific meets the Sea of Cortez):
La Paz (Sea of Cortez):
Mazatlan (Pacific):
Playa del Carmen (Caribbean Sea):