Upon triggering, the area seems to darken and expand in all directions, so that the exits are no longer visible. High winds start up, and sea water seeps up from the floor, rapidly filling the area as far as the eye can see. Clouds form overhead, and soon the characters appear to be out in the open ocean, with no floor underneath.
After a moment, a ship's mast rises from beneath them, then they feel solid footing underneath. A sailing ship comes up from the depths, and they are suddenly standing on its upper deck, with manic crewmen running about, shouting about pirates.
Barreling down on the characters' ship is a menacing vessel with a Jolly Roger flying from the main mast. The characters now have a battle on the open seas, fending off pirates. If they fail, they may be killed in the fight, or taken captive. If they are successful, they may take the pirate ship, or drive it off.
Either way, upon winning or losing, any ships still afloat slowly sink back into the water. Then the water retreats, the clouds fade, and the characters find themselves standing back in the same place as before. Anyone killed is still dead, but their body is present.
Optional: This is all an elaborate illusion, and may be disbelieved with a successful Saving throw. Spellcasters may be able to dispel the illusion, using various magics.
Optional: This is very much not an illusion, and the characters may be able to retrieve an artifact, map, or clue germane to their current adventure.