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Given the location of Ensenada within Mexico you will be entering territorial waters of Mexico. In which case you will be going through immigration on your way back to the US, so if you have a single entry visa and currently in the United States you will likely not be admitted back into the country unless you have a travel docs
For more details you can take a look at requirements and recommendations from cruise lines like Princess. You may want to check if you will be able to get back in with an Advance Parole form