Hah! You and me both. I've been kicking around ideas too...
First up, I would absolutely love to play a maritime based game, and have a marine nearly built just waiting to join.
I do think you should define what exactly you're looking for - out and out pirates? Shadowrunners on the water doing piratey things? Water based mercenaries? Makes bit of a difference. Also, where in the world - interfering with the war in Amazonia? Bobbing around SE Asia? Bad deeds in East Africa? There's plenty of choices...
Level? I'd go with the starting book, unless you have a strong inclination for more or less power. Only slight difference I'd make is pirates need a lot of money, as the big transports are not optional, so maybe be kind there and give a little more. Maybe just give a slush fund to the whole group for a ship.
Starting? Again, nothing wrong with the classic 'Mr Johnson/Fixer's got a job to offer to all of you', although an alternative could be survivors of a law enforcement/pirate attack having to work together. I'd probably go with the former.
Personally, I'd go with SR/Mercs on the water (anywhere you can do piracy tends to be fairly rough) - so sometimes stealing cargos, sometimes hunting down pirates, sometimes delivering valuable goods through pirate infested waters, the odd piece of hostage extraction, watching an Atzlan convoy on behalf of Kane and so on. Keeps things nice and varied. And vary missions between out where there's no law, and anything goes, and going back to town to flog the gear and then suddenly being asked to bug the Duc d'Enghien's luxury yacht prior to his soiree tomorrow night...
Well. That's how I'd do it. Hopefully that gives you some ideas.