While camping at Big Bend NP this Christmas a friend and I crossed over to Boquillas via row boat. There's an American Border Station on the US side and a Mexican Border Station on their side. We had to show passports (or passcards) on both sides. The US side was probably just to make sure we had what we needed to get back to the US. On the Mexican side we were escorted to the Mexican Border Station, had to show our passports/passcards and had to fill out forms for our visit. Prior to returning back to the US we had to surrender those forms. I don't remember, but that form may have been a simple Visa for our short visit for lunch.
The friend who went with me on the crossing goes to Mexico all the time for his work and he was very familiar with the process and the forms.