Masked Gunmen in Mexico - Pirates or Police?

We just recieved this report from our YachtPal Dennis Gade on S/V Dolce Vita, warning cruisers about the men in black masks who approached his sailboat with machine guns and assault rifles while anchored in Bahia San Quintin, Mexico.

"It was during the morning and we had just finished a great breakfast of eggs and bacon with coffee. Bob tells me there was a small boat coming our way. I told him they probably were fishermen. As they came closer, they began to circle another boat near us, but they were still too far off to make them out and see if the figures we saw them holding were fishing poles or guns. Yes, I said guns. After investigating the other boat, they started heading for Dolce Vita. It became very clear at this point that they were not holding fishing poles, but rather they were holding machine guns and assault rifles. There were six of them in the boat, one of which had a black ski mask over his head and face."

Luckily Dennis had just read a cruiser's report saying this was a new way for the Federales (Mexico's Federal Agency of Investigation) to come say "hi." He greeted them with a smile, they asked him and his crew, Bob (but not Wilson), to sign a piece of paper, and then left. Presumably to go terrorize the next boat.

So, how is a boater in Mexican waters supposed to know the difference between a boatload of well-armed cops and a boatload of well-armed pirates? Your guess is as good as ours!