This fleet is based loosely on ziotoo4's "Incredible But True fleet". Well, sorta... it uses a canceler to get around the "can't dock at your home island" part of the Mercenary keyword. In ziotoo4's fleet, he used Mobilis in an all-submarine fleet (neat idea!). Here, I'm using the cheapest canceler in the game (Lenoir from Fire & Steel, a piece I don't even own yet) on the cheapest French ship in the game (Le Coeur de Lion).
I'm also trying to take advantage of the Ex-patriot keyword, specifically the +1 bonus when rolling for a Mysterious Island effect. Normally, Merc ships and their linked crew are just not worth playing... slow, too expensive, and too many abilities that just aren't useful. There are at least two ship/crew combos that don't completely stink.
First, there's the only Merc ship that could be called a decent treasure runner, even if it's 12 points: the Matuku, with Luc Savard. She's the fastest Merc ship with S+L speed, 4 cargo, and three 4L cannons. Luc adds a Captain ability and a +1 vs. American ships. Add an explorer, and she's a decent treasure runner that can fight if need be.
Our second ship is the Santa Molina, probably the best thing the Mercs have that could be properly considered a gunship. Slow, so we need to add a helmsman, somewhat average cannons, and a much-appreciated crew-killing ability. Linking up Osvaldo gives us the Captain ability and Broadsides for only 7 points, which is nice because his cost doesn't appear to be inflated by 1 point as compared to similar abilities in other factions. Broadsides on a ship with only rank 3 cannons is more of a toss-up than an advantage, but then again it does counter certain defensive abilities such as El Acorazado's and ships with "can't be hit by L cannons". It has two cargo open, so it can grab some treasure if need be.
This fleet is designed to work with Mysterious Island effects, since both the Matuku and Santa Molina get +1 on their rolls. Chances are really, really good that your opponent won't have this advantage, and with a bit of luck on your side, hopefully he'll get burned by Mysterious Islands more than you will. For a list of Mysterious Island effects:
http://forum.wizkidsgames.com/showpost.php?p=1587923&postcount=3
For this fleet, I recommend multiple copies of MI#1, MI#6, or MI#15. If you have time, move away and redock repeatedly. MI#1 will relocate all unclaimed coins to the island you're exploring. Take the high coins (hopefully enough to win), plop a fort on her, and head home. MI#6 is for brining in multiple events. Start with False Treasure, then follow up with Mermaids, Foul Winds, and Becalmed. If your opponent is using events against you, then bring in Favor of the Gods. MI#15 is another way to bypass the "no docking" Merc keyword by transfering gold directly to your home island.