How else are you going to get out of a country surrounded by water? At least w/ Canada there's an agreement for the Canucks to tell CBR that you've left the US

How else are you going to get out of a country surrounded by water? At least w/ Canada there's an agreement for the Canucks to tell CBR that you've left the US (Source: I tried to surrender my i-94 at the US side of the border. The guy told me to give it to Canadians). Not hard to envision another agreement on the Mexican side