I've got an idea I'm fleshing out for an adventure but I'd like some help fitting it around the lore of D&D.

The first part of the story works fine, but the twist happens when a player opens a chest in a wrecked ship, and suddenly they find the ship restored around them and floating at sea - so far, easily explained by "magic". However, the plot revolves around the idea that the mountains around where they were have become islands, as if the sea had risen by hundreds of feet, and that's where I'm struggling to tie it in.

I don't want to railroad, so I can't just say "it's a magic place and you can't get out until you complete the story", so I'm trying to work out where it could fit, so that if players have the means to navigate away from it then they can. In reading, the Feywild could be an option, as things are extremes there so an ocean might have become higher. Are there islands in the Plane of Water, or is it endless water in all directions? If so, then this could be a viable location for them to be taken to.

Any feedback welcomed, thanks!

I've just done some research and found out about Demiplanes, which appear to cover my requirements quite well. It also would allow for predefined rules for navigating out of it, in addition to the ones I intend to use as story features. Would this suit properly, do you think?

Yes indeed, in fact a campaign that I'm playing in hinges on a similar concept: the players are trapped in a demiplane, needing 3 artifacts to get out.

Brilliant, thanks! I'll get back to writing it with renewed vigour!

I would like to offer another method of "explaining" this phenomenon.  What if this area were rising or falling into the sea?  If it is rising, this magical event could be going back in time at the same location when the ship was still intact.  You would have to adjust things to make it agree that this is some time ago, so there would not be evidence of some cities, roads, or other things that "developed" during the passage of years in between.  If the party were to use Teleport, they might find themselves at some random place because the "Teleportation Circle" has not been magically created yet.

Alternatively, if the area is sinking into the ocean, then you would have the ruins of cities, roads and stuff that the ocean now covers up.  So if the area is sinking you would be in a future universe.  If the party were to use Teleport, they might find themselves underwater when they "arrive" because the Teleportation Circle is now under water.

Quote from MusicScout >>

I would like to offer another method of "explaining" this phenomenon.  What if this area were rising or falling into the sea?  If it is rising, this magical event could be going back in time at the same location when the ship was still intact.  You would have to adjust things to make it agree that this is some time ago, so there would not be evidence of some cities, roads, or other things that "developed" during the passage of years in between.  If the party were to use Teleport, they might find themselves at some random place because the "Teleportation Circle" has not been magically created yet.

Alternatively, if the area is sinking into the ocean, then you would have the ruins of cities, roads and stuff that the ocean now covers up.  So if the area is sinking you would be in a future universe.  If the party were to use Teleport, they might find themselves underwater when they "arrive" because the Teleportation Circle is now under water.

 Thanks for the ideas! I can't really use the going back in time idea as this conflicts with the rest of the story behind the adventure, so I will probably be going for the demiplane approach. I want this to be an encounter/adventure which can easily be dropped into any mountainous terrain, so having it sinking or rising is quite a big thing to have to add to the world!