I draw a lot of inspiration from history and from books and movies. But I think what helps me the most when writing adventures, is reminding myself about the central themes of my campaign.
Theming
For my current 3.5 pirate campaign, I chose water, pirates and exploration as the main themes. And this informs every choice I make when writing adventures. It determines what kind of monsters I use, what kind of environments I set my adventures in, and what sort of cultures the players will encounter (which draw heavy inspiration from real historical pirates).
Whenever I come up with something, I ask myself: "Am I subverting the expectations of my players?"
This is very important to me. I do not want my adventures to feel like just another dime a dozen D&D adventures. Water is what makes my current campaign unique, and so I try to incorporate it into everything. I also put myself in the shoes of Walt Disney and I say to myself: If this was an amusement park, would this amaze my players? Would they be struck with that feeling of wonder and awe that I want to inject into my adventures? Or would it be just what they expected?
Environments
When my players step into the world that I've created for them, I want them to feel like there is wonder and awe at every turn. I want them to see and experience things that they would never find in any other campaign. For example, if I have an adventure that takes place at a tower on an island (as I recently had), how will this tower amaze them, and subvert their expectations of it? What makes this tower unique in a way that you could only experience in a water-themed fantasy adventure?
My experiences from traveling help me flesh out the finer details of the exotic locales that are featured in my adventures. The OP's article mentions a watery cave, but what does a cave like that sound like? Is it hot or cold? What sort of rock formations would you see? What does it smell like? These are the sort of details where you can lean on your own experiences from visiting actual caves.
For example, when my players were exploring a flooded cave, I described to them how they could occasionally hear a deep loud bang in the distance; almost like the sound of drums. It was the sound of a boat hitting the side of the cave, magnified by its echo in the vast underground complex. These are the sorts of details that bring the location to life for your players. It really helps with the immersion if you've actually been to such a cave, and get the details right.
Characters
I try to write my npc's in the same way that I write locations. The players may have some rough expectations of what a character is going to be like, and so I try to subvert those expectations, by writing a character that is not at all what they expected. Just as I try to infuse the world with wonder and awe, I also write my characters with interesting motives, character traits, and world views. I want them to genuinely be curious to meet with the characters I create.
Naming
It is important for me that both characters and locations have names that are easy to remember for the players, yet defy cliches. In my adventures you will not find cities named after weapons or containing the words death or shadow. I try to follow real life naming conventions. If for example I have a region that is inspired by France, then villages and cities will have French names that stay true to how such places would be named in France. They may be named after local mountains, rivers and castles. And npc's in that location will also have French names, which occasionally refer to their occupation, or a character trait. I'm also a fan of alliterations, and will often give characters a first and last name that have a sort of lyrical quality to them. There's no point in giving your villain a super complicated name, if your players will not remember it, nor know how to spell it. I took some inspiration from George RR Martin, by giving many of my characters a nick name that is easy to remember. So you don't have to remember the name Prince Oberyn or Sandor Clegane. You can just remember the names The Red Viper and The Hound.
Encounters
The same rules that I use for designing locations, also serve me well when designing encounters and battles. I will use aquatic monsters, or opponents that make use of aquatic spells. I will design my encounters around aquatic terrain of many varieties, and I'll try to incorporate water hazards when ever I can. What would be a dungeon crawl through a default brick labyrinth in any other campaign, may be a crawl through a sinking pirate ship in mine.
Narrative
But I'm careful not to let one central theme dominate each and every adventure. Sometimes it just does not fit. Not every element of each adventure had to involve water directly. It could also revolve around the central theme of the campaign indirectly; such as when the players were approached by a monk, who asked them to steal an ancient religious manuscript, detailing the oceanic voyage (and demise) of an important saint. Such an adventure still uses ships and the sea for its backstory, while taking place entirely on land. Of course stealing the manuscript, involved breaking into a half-flooded crypt. And there we're back to the theme of water again.
I try not to cling too much to what I write though. As the players make unexpected choices, I may feel inspired to change a character's motivations, or the overall story line entirely. Do not be afraid to let your players inspire you and don't set too many details in stone. Be willing to adapt as the story finds its own course.
