This encounter can be sprung on the PC's at any time while they are traveling by sea in relatively shallow waters. Alternately, if the PC's have participated in the events of the scenario “The Message,” they could even be working for Don Santiago with one of his dive teams. All characters who are in a position to do so should make DC 20 Spot checks; those who succeed notice the shadow of the wreck beneath the waves.
Given that the water isn't too deep, hopefully the PC's feel some desire to explore the wreck. Assuming that they or the captain bring their ship back around to it and drop anchor, they are in position to do so. At that point, they could face any number of difficulties and dangers in making their investigation. Detailed here is just one way in which this situation could develop.
*DC 10 Swim checks are required to move about in the water. Characters can give themselves a +5 equipment bonus if they used diving waits to make their descent, although these inflict a penalty if not removed before surfacing.
*At the same time, the GM should keep track of how long divers can hold their breath, a number of rounds equal to twice their Constitution scores. After that, they must begin making Constitution checks with a DC of 10, increasing by one for every subsequent round. Failure means that the character falls unconscious in the first round, declines to -1 hit points in the second and dies in the third.
*The wreck could contain any number of creatures. For example, a squid or a swarm of crabs might inhabit the wreck, and combat—leaving blood in the water—could attract a shark.
*What exactly the PC's find in the wreck is up to the GM, along with its ship type, name, and point of origin, and the identities and backgrounds of its crew and passengers. This is a good chance for the GM to introduce some intrigue, if there is evidence of any kind of mystery to be found.
*For example, a DC 12 Search check could reveal, in the captain's cabin, a body impaled on a finely crafted and intricately engraved rapier. A DC 20 Search check reveals a locket around the body's neck, one that bears a pair of initials.
*There is also the matter of recovering any loot or other cargo aboard the vessel. This could include chests of coins or other such valuables, but is more likely to entail trade goods. Whatever the case, the PC's should make Knowledge: engineering, Profession: sailor or other relevant skill checks to devise a means of raising such heavy items. A good option here is to detach the ship's anchor and use the capstan for a mechanical advantage.
As usual, the GM should feel free to tailor any and all details of this encounter according to the needs of the campaign and the desires of the players.
This interlude provides plenty of opportunities for continuing adventures; listed below are only a few of the possibilities.
*The cargo stowed aboard the wreck might contain a valuable sought by a relentless and ruthless individual. If the PC's take possession of it, that person might come looking to reclaim it.
*As mentioned above, the wreck could also contain clues to a murder. Should the heroes decide to investigate the matter, it could lead to a lieutenant who betrayed his captain and is perhaps even making a move on his superior's young widow.
*If the crew of the unfortunate vessel had unfinished business of their own, the PC's might find evidence of it and even decide to complete the task.
*Perhaps enemy pirates (and even including a vengeful sea witch) are behind the sinking, and don't take kindly to the PC's absconding with what they see as their rightful booty.