Jeff,
as I understand what you have said about not being able to go "underwater" except in P3D it might be appropriate to but a disclaimer notice in the readme.txt telling people it is P3D only scenery.
They can certainly load this scene into FSX if they want to. They can go and see the trawler, orcas, jumping fish, and the dive captain. But they can't swim with the whales or explore the wreck.
Here is the first line from the readme that I included in the zip file-
"A diving scenery created with Instant scenery 3 by Jeff Greene for Return To Misty Moorings in Prepar3D."
This thread is the only place they could possibly get this file from. And it is dealing with diving in P3D. They've seen me working on it in the pictures above. See reply #14 above also.
But I can amend the readme to include a disclaimer if it is still unclear.
Edit: Readme file has been updated.