Mexican water rights were considered separate from the underlying land rights unless they were explicitly included in the grant. Moreover, Mexican title was required to be registered with the government shortly after annexation. Most of those titles were in turn siezed by various quasi-legal means, which is where cities like Berkeley come from. There are relatively few water rights remaining from Mexican annexation, most of which are held by municipal institutions like LA and only affect relatively small streams. Those of larger areas, like the Sangre de Cristo grant, have been litigated to death in courts over the past couple centuries and most of the entities involved no longer exist.
Water law is a nightmarishly confusing hellscape, but Guadalupe -Hidalgo isn't an important reason why today.
