This goes along the lines of finding floating debris. Instead of wreckage or junk, however, a bottle is discovered, floating in the water. This could just be an ordinary bottle, of clear, colored, or opaque glass, and it may contain a clue in a mystery or small item. It could have an S.O.S. or other sort of message inside, or a treasure map, or something more exotic, such as a bound spirit, demon, or djinn. Perhaps a wizard has cursed an entire ship, shrinking it magically and placing it in the bottle, and then casting it out into the ocean, doomed to float about at the mercy of the tides until such time as it is discovered and some means of rescue is devised. At this point, the PCs may have a new vessel in their fleet, or else great gratitude and reward from the sailors, should they still be alive. This may also be an effective way of introducing a spelljammer vessel into the campaign, if one uses that setting.
It could also be a magic item... suggestions for an bottle of endless smoke or endless water might be good (if it's a particularly ancient decanter of endless water, this item could in fact be the source for the body of water the party is currently travelling on... if so, they may need to find a way to shut the water off, so that the flooding does not cover the entire continent. Of course, shutting off the water source may also result in major geographical changes over the long term (if all the water runs off from a large, artificially-created river or lake for example, revealing an unexplored bed, possibly with all kinds of ruins to be discovered... along with deep mud