The guy in the white shirt at the bottom of the ladder is the harbor pilot for Mazatlan. His job is to board (and disembark from) vessels heading in and out of the bay. This was the La Paz ferry and I saw the tiny little door with the Hotel flag (white/red) open.
The pilot boat zooms underneath and pins itself to the moving ferry, and the harbor pilot scrambles down. In some ports they actually pick them up and drop them off with helicopters.
Harbor pilots are worth a read if you've never checked out the profession. Their job is all about handling huge ships in tight confines and doing it perfectly, not to mention having the physical prowess to hop from one moving vessel to another or from aircraft. It's an old job as well. The earliest harbor pilots on record date back to 700 BC.