Here is a really off-the-wall idea inspired by the movie Star Trek II – The Wrath of Khan. The basic setup runs as follows:
Two Sailing Ships
One that is a smaller, faster ‘Cutter’ type (say a Longship) under the control of the baddie and one that is a larger ‘War Cruiser’ type (Galley or Warship) carrying the characters.
An evil Wizard (or similar type of character such as a Cleric serving their deity) with dreams of power and glory has a magical object that they have crafted or found after a long quest. The object is capable of ripping a hole in the fabric of the Prime Material Plane. They could be trying to ‘access’ the Elemental Plane of Water, the Nine Hells, or another plane of your choice so that pure and utter chaos spews forth. The real twist that brings in the sailing ships is the need to have the object in just the right place geographically when the stars and/or planets align. This could be a point at open sea, an island with a ‘lost’ temple, or a unique location of your choice. Proper placement is necessary to activate the object and rip a portal/opening into existence. Obviously, the NPC in question thinks they can control the outcome or will receive a great reward from the ‘powers that be’ on the other side, but it is pretty much a suicidal pursuit. The object is going to destroy everything within a set distance when it activates. The characters are facing a time crunch to stop the baddie from succeeding with their plan (the object will activate at a set time of day or night once in position). Can they find the rogue ship in time and stop the baddie from activating the object? What will they do if the baddie succeeds in activating the object?
1.) You could even go so far as to have a severe bank of fog in the area for the ships to hide in, thus replicating the nebula battle portion of the movie. A fight between the ships in the fog bank could be done via magic spells such as Fireball or weapons fire like flaming arrows, etc. 2.) If you were looking for an epic end battle, you could have unexpected help appear in the form of Merfolk or other ‘citizens’ of the sea joining in to help save the day.