There is a saying that "To steal ideas from one person is plagiarism; to steal from many is research.". When creating a campaign for Dungeons & Dragons, most DMs borrow ideas from one or several sources. I once played in a homebrew The Witcher D&D campaign, and when I recently watched The Witcher on Netflix, one of the episodes was pretty much the one adventure we played in that campaign. Apparently both that campaign and Netflix based it on the same short story.

I presented my idea to build a campaign together to my players, and they suggested a nautical / aquatic campaign. That immediately solved the question of what material I could steal: WotC has last year published Ghosts of Saltmarsh, a nautical campaign setting and adventure collection. And I actually bought the Sinister Silver Edition from Beadle & Grimm's. And I participated in two Kickstarter campaigns, one called Depths of Savage Atoll for .stl files for aquatic / nautical / pirate themed adventures, and another called Sea King's Malice, a 5E adventure with the same theme.

In the early days of the internet, TSR, the company that had created Dungeons & Dragons, was known for going after people who published self-made D&D stuff on Usenet. That was never a good idea. After WotC bought TSR in 1997, they changed that policy, and in 2000 came out with the Open Game License for 3rd edition D&D. 5th edition has its own OGL. Third parties can take the System Reference Document SRD of 5th edition and publish stuff compatible with that, and as the SRD is just 5E without the copyrighted names, the third party product will thus be compatible with 5th edition Dungeons & Dragons. On DriveThruRPG and their sub-site Dungeon Masters Guild you can now find thousands of 5E compatible adventures and source books. On DMsguild there is even a "storyline" search term, where you can find all the materials related to a specific WotC book, like Ghosts of Saltmarsh (as the time of writing this, there are 100 adventures related to Ghosts of Saltmarsh there). I might actually already own some of them, as I tend to buy Humble Bundles of 5E adventures on the cheap.

In short, I am pretty confident that I can create a good nautical campaign for my players. The ideas I'll end up using might not all be original, but as long as they are new to my players, that doesn't really matter. In the next session I'll ask them some more questions about how they imagine the campaign. Do they want to be heroic adventures battling evil (which is what Ghosts of Saltmarsh assumes) or rather a group of pirates looking for power and riches (which is what I suspect they'll prefer)? Just like this Ghosts of Saltmarsh playthrough from How to be a great Game Master, I suspect the final result will be substantially different from the source book. Good DMs borrow, great DMs steal, but the final result will be somehow our own.