I like Melvin's twist - occasionally the entire boat is taken as an offering :)
And yes, there is something in the water - perhaps a dragon or similar beastly. It would have sorcerous powers so it would occasionally gift good luck on the donors.
Lots of plot hooks can come from that - a jealous nearby town might try to end the local town's luck by any number of ruses - perhaps poisoning the lake, sending warriors to kill the benefactor, dropping trapped gifts to enrage the beast, etc.
Even nobels could get jealous and seek to know the truth, and perhaps capture the beast.
I expect the purpose of this encounter is to teach the lesson of learning about places you are going to go, as in most circumstances freeing the victims is the 'correct' behavior. Given the lead in is the PC's have been led off the beaten path, this is not likely to have happened, so the PC's either will have done something normally immoral (leave em to hang), or they will be led into a possibly lethal encounter with a tribe over cultural differences.
I'd expect _some_ type of foreshadowing to hint at the correct behavior.
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Very fun, especially if the Gm seeds some red herring magical items among the latest trove of treasure the group think *may* be cursed.
Turn dead spells to banish ghostly saboteurs, see the invisible to spot the meddlesome mage, and accusing the thief of going overboard to prove her usefulness to the group. All sadly in vain for everything keeps coming full circle, even that rotting sandwich, formerly silently moldering in the bottom of Stella's travel pack until she noticed the repulsive smell...
So much fun to be had by the inclusion of one innocent little fey....