Professional gunmen doing what exactly, while they are on a go-fast boat or narco-submarine with actual professional gunman on the docks on either end? The only people capable and willing to board such boats are the coast guard. Sending higher up professional gunmen to risk the open seas would be virtually pointless. Even if a private individual/group tried it, the cartel would 'deal' with them as soon as they got it back on land, no need to risk their enforcers on the high seas for such an improbable event.

You can see the same thing watching drug trafficking police videos. Most the time the drugs are in transit it is some random poor person with not much else going on in their lives. As soon as they are confronted by armed men they play dumb until the ruse is up.