Having gotten the party onto the Island, we must do something with them. We can safely assume that they will have little interest in the duties of the other ship mates - they might help, if asked, but I wouldn't count on the party having much interest in collecting water and firewood or in hunting.
We could always hand the players the adventure on a silver platter: "You see a large, crystal shrine rising fifty feet above the trees, with a tremendous sign, Treasure Buried Here." I once played in campaigns with other youngsters where this was about the game's speed.
Barring this, however, it will not be very long before the party utters those dreaded words, "We search the island." It may surprises some, but for many the act of describing an island - even a small one - sector by sector becomes a tedious process.
A typical DM will cut right to the chase. What is the point of dragging it out? "The party finds a cave where the surf pounds the cliffs on the west side." Simple, straight to the point, let the dungeon begin.
After all, how is the party going to search the island? Inch by inch? Foot by foot? Do they stroll about at a normal pace, like any of us would searching an island like this, enjoying the sights and sounds? It's quite unlikely that they'd see anything hidden or unusual that way, since the island is obviously regularly used by ship's crews. And what is the point, anyway? Sooner or later the party is going to find that cave, so all we're doing by having the party wander about the island pointlessly is wasting time they could be spending in the dungeon. Right?
Well, a DM could argue that the party ought to put in some effort. They ought to try a little before finding the cave. And if they don't put in the effort, well, tough luck. If only they had tried - but they didn't. So sorry, all, no dungeon for you!
This has even less point. Now the DM has spent time creating the island (and, presumably, the dungeon), only to have a moment of useless superiority by denying the party everything. What is the party going to do now? Wander off to another place, presumably, where they DO find the dungeon, so that gaming can result. Wouldn't it be easier to let them find the dungeon on the island?
Here is a point to consider - why should the party even suppose there is an adventure here? Unless the DM makes it obvious to the players that there IS something on the island, it's reasonable for them to think, "Oh well, this is just an island, no reason to presume something is happening." And parties will do this.
Every party that finds a dungeon entrance and starts inside assumes that something is going to be down here . . . but I've found that most players on roads, moving through woods, landing on islands, poking about a village and so on will assume that this is just a normal day. The reason is obvious - if every 10 mile stretch of road was an 'adventure' like a dungeon, the party would never get anywhere. If every village was another Hommlet, characters would be in their rights to lay waste to the countryside. Thus are wildernesses empty, while dungeons full of promise.
Ask this, O Reader: how does a film script convey to an audience that there's more than meets the eye? Yes, that's correct. Clues. An imperceptible movement, catching the character's attention but proving to be a shadow. An unusual object lying on the path. The movement of animals. A cliff that looks distinctly odd when viewed from a particular angle. An distinctly unnatural sound. Fit in nine or ten of these little details and a party will begin to think there's something up - without having to put up a big sign.
Suppose we break the island down into its requisite parts. A ridge along one side of the island. An opposite hillock. A central lagoon. A bridge of land connecting the two parts of the island. A spit that nearly cuts off the lagoon. A passage of water connecting the lagoon to the sea. A small rocky islet. A small pillar and a large, tall pillar. Two distinct cliffs, one at the top of the picture, the other bottom left. A steep shore that runs below the ridge. The aforementioned well & windlass, where water can come from. Here are plenty of things from which we can make clues.
The well-water tastes metallic. There are large bubbles rising in the water on the shore below the ridge. The cliffs at the end of the ridge are too sheer to climb, but there is the picture of a seven-foot humanoid scratched there, ten feet above the waterline. The lower cliffs are stepped and easy to climb; these are popular with birds, who for some reason become absurdly spooked without apparent reason. The pillars are covered with moss. The islet has a black, indistinct shape on it, that can't be made out. The passage connecting the lagoon to the sea is littered with stones on the bottom that twinkle in the sun. The spit seems to have been worked by stone masons, but only in part and for no apparent purpose. The bridge between parts of the island gives a feeling of unease. The central lagoon is no more than four feet deep, the bottom covered in sand; but where the sand is rippled in some places, in others it is smooth and flat.
More than enough things to grab our attention. Naturally, the party will pull out wands and spells for detecting evil, enemies, life, treasure and what have you, but its easy enough to remind them that the distances on these things are usually inadequate. The lagoon is at least 200 feet across; it's full of little darting fish and algae. If there's a giant crab buried in the mud, the character with the right means of detection would have to be standing on the right stretch of shore, no?
That could also be a monster laying in the water off shore, making bubbles. But where, exactly, how far away, how big, etcetera? A sea snake? A giant squid? A natural spring? The party may have to dive in to find out.
Why does the moss that grows on the two pillars not grow elsewhere on the island? How hard would it be to swim out to that little islet? Bad waves, no doubt, a strong current, rocks hard to grasp once the islet is reached . . . a party would have to work these things out. And what is that disturbing the birds?
It should be understood that all of these clues lead somewhere. It's tempting to make them a lot of red herrings, but let me remind the reader: in a dungeon, we make every door go somewhere. We're responsible for doing the same thing in the outside world.
So, what shall we find? Well, I leave that up to you, gentle reader. Let me know what you'd like to investigate first - and we can give it a look-see.