There's some good stuff here azato! I just ran an island adventure, then pirate ship-to-ship adventure, and I'm going to run a third adventure today combining the best elements of each, fine tuned by what's I've learned play the other two.
You've developed some good ideas...I wish I would have had them when I ran my original shipwreck adventure, but I will mention one thing, as iserith said, you may get a player who criticizes your whole effort by saying, "If you wanted us on the island you should have just started us there." That's what happened to me. I thought a shipwreck start would provide dramatic action and tension by way of chaos, but the criticism was, starting with a shipwreck is "railroading" b/c the players choices don't matter, they are ending up on that island no matter what. I was told such "railroading" goes against the Agenda & Principles of DW.
So, I came up with an alternative idea for getting a ship's crew on an island. It invovles ticking off Grim Portents. The PCs are sailing a ship near an island (a nautical landmark). When they round the island...they see a sail in the distance (GP1), the pursuing ship closes the distance (GP2):
FRONT: Ship-to-Ship Battle
DANGER: Pirate Ship
Impulse: plunder anything of value
GRIM PORTENT: You see a sail in the distance
GRIM PORTENT: A ship in pursuit closes the distance
IMPENDING DOOM: Ship-to-Ship battle
Now the PCs ship is engaged in ship-to-ship combat off the shore of the island. Start ticking off Grim Portents again...for each ship:
DANGER: Pirate/PCs Ship
GRIM PORTENT: combustible catches ship on fire
GRIM PORTENT: fire burns the sail irreparably
GRIM PORTENT: explosion punctures hull, ship taking on water
IMPENDING DOOM: ship sinks
• Grappling hook to connect ships
• Swing over to other ship
• Climb to crow’s nest to snipe
• Hurl combustibles
The drama here is in the back and forth tit-for-tat tick down of Grim Portents. Who's ship sinks first? Do they both sink? With enough PCs and enough combat on deck (defy danger to swing over or climbing to snipe, melee, lauch combustible), both ships will quickly go down (they did in my ship-to-ship adventure).
Now you have all the survivors attempting to swim to the island…PCs and Pirates. Do the pirates become yet another Danger on the island, or do the islands Dangers present so much trouble, the pirates join in an uneasy alliance with the PCs to try and escape the islands Dangers together?
The number of PCs being presented with opportunities to roll dice during the ship-to-ship combat will more than likely bring the Impending Doom to pass for both ships, marooning the PCs on the island…which is your goal. It also puts it in the hands of the PCs dice rolls. Meaning their choices dictate the outcome – avoiding any “railroading” criticisms. However there is the possibility that all your PCs will roll dice like total rock stars, leaving most of your Grim Portents unchecked…i.e. no shipwreck. What then? So look to the advice iserith gave you as the safest risk-free way to go – if you want them on the island, start them on the island.
Me myself…I dare to be exciting! I’m a bold risk taker. I go for it ;)
Good Luck. Let us know how it goes. My guys love this pirate stuff! I have a feeling I'll be running pirate adventures for a while ;)