Just making this up on the spot:
The players are treasure hunters on a small island. They are trying to find the location of a shipwreck, said to carry a vast amount of wealth. The players all need this wealth to settle various personal financial problems, which they can think up for themselves. (or you can offer them various backstory choices for this, and then weave those into the campaign)
They have a shady contact in town, who knows of a way to locate the shipwreck. He points them to a local church, which has many depiction of the ship. This contact will set up a meeting with one of the monks of the church.
This monk can give the players access to ancient writings in the catacombs underneath the church, but he is doing so without the permission of his church. He is willing to make a deal with the players, because he is being blackmailed by their contact, for secretly breaking his vow of celibacy. So under the cover of darkness, he lets the players into the church through the back. He gives them directions to the entrance of the catacombs, but they must do so stealthily, and without doing any harm to the other monks. If they are spotted, the monks will sound the alarm, and call the city guards.
The catacombs
The catacombs are flooded, due to a heavy downpour a few days ago. The players will need to do a lot of swimming, and will probably have to deal with some low level undead (of course!). In one of the chambers the players find an ancient mural that details the voyage of the ship and its mysterious cargo: a strange coffin, along with a lot of valuable relics of various saints. It seems the crew of the vessel, which consisted entirely of priests and paladins, intended to drop the coffin into the ocean at a nearby island. But something must have gone horribly wrong. With this information the players can now track down the shipwreck. But they'll need to charter a ship first. At the docks they'll be able to hitch a ride with one of the merchant vessels. Their contact will gladly join them on this journey, since he wants a share in the plunder. This also gives you as a DM a means to give information to the players, through an npc who joins them on their travels.
(For a group of beginners, it can often be helpful if the DM has an npc tag along, so the players don't get stuck. This npc can provide ideas for what the options of the players are. Just be careful that this npc does not become a substitute for the DM himself. Preferably, this npc shouldn't solve the problems of the players, nor take part in any battles. He should be an ally, but not a crutch. And above all, he should not have plot immunity. If he dies, then he dies.)
The shipwreck
Once the players have found a way to reach their destination, they'll have to find the shipwreck. They'll need a rowboat, and then dive into the ocean to seek the ship wreck. They'll have to deal with sharks, which are fair opponents for a low level party. Once they find their way into the hold, little of the treasure is still there... but it seems the mysterious coffin has been opened, and the coffin is empty. Their contact is only concerned with the contents of the coffin, and not with the treasure. He explains that the priests on board this ship brought something of great power in the coffin. If they can uncover where they found the coffin, they may be able to find far more of the treasure as well.
They also notice that the lid of the coffin has some ancient writing on it, that none of them can read. Their contact suggests that there may be someone on the island who is able to decipher the writing, at the local library. The librarian is able to translate the text, and reveals that the coffin was found in an ancient temple on the island. The island has a long history, and there are still plenty of ruins from an ancient civilization on the island. He will make a paper copy of the writing on the lid, and then attempt to have the players and their contact killed, while he makes a run for it. As it turns out, the librarian belongs to a secret evil cult dedicated to uncovering the secrets of the island.
And thus the players quickly get involved in a vast conspiracy where various secret groups are trying to get a hold of the contents of that coffin, and whatever it may have contained. And this all leads up to a secret that revolves around the whole island, or perhaps even bigger than that. You could even throw in a twist at some point, where their reliable contact also turns against them. There's enough room here to fill several sessions.
(The shipwreck itself can be a mini dungeon. You can increase the excitement, by having sharks follow the players into the wreck, and creating some truly terrifying James Bond-type scenarios. If the players do not have access to a water breathing spell, then you can also include small pockets of air inside the wreck. This may not be realistic, but it's a game after all.)
