That would be the Mexican border patrol, since the US border patrol cannot enforce anything on the other side. They would treat the incursion very seriously. They would likely assume you are dangerous, a mental patient, and/or a terrorist. You would be in their custody until they figured out which ones you are, and you would be at severe risk of going to a Mexican jail. This is something you never want to do. Many Mexican jails do not even feed prisoners, instead they are expected to have a family member bring food, clothing and other necessary supplies daily. Unless you have someone who can fulfill that capacity for you, you will be at the mercy of their system. That said, before 9/11 I took a rowboat over from Texas' Big Bend National Park to a tiny Mexican town named Boquillas. It was common to take this day trip, have lunch and a wonderful Mexican Coke made with real sugar, and rowboat back. Recently this rowboat crossing was re-opened with border patrol agents using remote technology. (Link: Border crossing in Big Bend breathes new life into tiny town Boquillas)I've also gone river rafting on the Rio Grande, again before 9/11, and the rafts sometimes stop on either side of the border for breaks, lunch, etc. I presume these rafting companies still do this. I believe we carried our passports on us in case we were confronted.