Here's a pure Mercenary fleet that actually won a game!
The Santa Molina is the flagship. She can dish out some decent punishment, especially with the right UT's. It's important for her to survive and protect the submarines, since Fiore is the only way to get gold home. The Santa Molina will often shadow the Barracuda to a nearby island, and let the Devil Ray explore a further WI.
The Devil Ray and Barracuda run gold. The Devil Ray will go long, with S+L+S speed underwater. It's quite possible that both subs might do some submerged ramming. In addition, the Barracuda's ability and cannons make her a decent support for the Santa Molina in a close battle.
The biggest strength is that each ship has some form of defense. The Santa Molina has a canceller, while the submarines will be submerged for most of the game, only surfacing to dock. Partway through the game, I realized that a shipwright on one of the two subs (probably the Devil Ray since she's faster) could be beneficial, instead of having the two one point crew on the subs. That's why I left it out there as an option (plus his look is just priceless so why not put him in the fleet lol). Originally the Barracuda had an oarsman, but the explorer can be left on the island if needed, although I dislike that rule. Also see the battle report for crew and treasure transfer shenanigans - Fiore and the UT's can make this fleet flexible under fire.