This is an idea that is dear to my heart and I'm looking forward to running a pirate/naval adventure myself. I've got some inspiration ideas for you!

Books

These books should need no introduction. These are the books that will truly inspire your game.

• Necronomicon: The Best Weird Tales of H. P. Lovecraft
  
• Eldritch Tales
  
• Conan the Barbarian - The Original, Unabridged Adventures of the World's Greatest Fantasy Hero
  
• Grimms Complete Fairy Tales
  
• Tales of Norse Mythology
  
• Iliad and Odyssey
  
  

  TV Shows
  

• Crossbones - This is one of the most underrated TV shows I've ever seen. It got canceled late in it's first season and so the finale was rushed, but despite that it's an amazing show. This show has political intrigue and adventure ideas abound. I'd argue it's damn near required viewing for running any sort of pirate campaign.
  
• Black Sails - This amazing show is written as a prequel to the novel, "Treasure Island". It follows Captain Flint, and a young John Silver as they attempt to make an "honest" living while preparing to thwart the predicted demise of piracy. Swashbuckling adventures, intrigue and more adventure ideas here than I can count. The show is a hell of a ride and I can guarantee you'll get plenty of ideas watching this one.
  
• Vikings - This show is the show that just keeps on giving. The first two or three episodes are a little slow to start, but you'll be on the edge of your seat every episode thereafter. While this show focuses on vikings, there's plenty here to inspire a pirate adventure. It will especially inspire the creation of your villains.
  
• The Musketeers - This isn't that terrible (and oh so amazing) Disney movie from the 90's. This show focuses on the Musketeers you know of, and the Musketeers as a military unit during that time period. If you're focusing on adventures during something similar to the golden age of piracy, you need to watch this show. One of my favorites.
  
  

  Tabletop Books
  

  
  

• The 7th Sea - I am not recommending this as the system you should use, I am recommending it strictly as inspiration for your setting. It easily has the most interesting setting I've ever seen and has some amazing ideas for adventures dealing with curses, the sidhe, naval campaigns, city adventures, etc. Whether this will be useful to you depends entirely on your setting. Are you playing D&D on the high seas? If so, this isn't going to be nearly as useful. Are you playing in a setting reminiscent of the golden age of piracy with some light magical touches (eldritch horrors in the deep sea, curses, magical fey creatures and low magic for the players)? If so, good lord is this book (and the Nations of Theah books) going to be useful to you.
  
• Razor Coast - If you're playing D&D on the high seas, this is the book for you. It's a sandbox setting with a ton of adventures and adventure seeds. It is expensive, but it's worth every penny. Highly recommended.
  
  I can't recommend all of this enough. A lot of this will give you ideas and inspiration for all of your campaigns, naval/pirate or otherwise. Have fun!

It depends on what you're looking for. The campaign guides will give you a lot of old rules that won't always translate very good into 5e, but might give you some ideas on table resources like special subraces or divine domains. The 3.0 Campaign Setting will give you a good overview on the whole continent, which can be helpfull to get a bit of a taste on what the different regions are like. The "Players Guide to Fearûn" of 3.5 will give you a bit more insight into the planes (although I'd use the World Tree and Blood River only as additional transistive planes on top of an astral plane and not instead of one) and the "Races of Fearûn" have a nice overview on the different species and subraces that the players might want to play, although it's not so easy to translate those rules into 5e. If you're interested in translations of at least 2 of the 4 additional subraces of Elves the "Races of Fearûn" has to offer, I've worked on the Wild Elves and Moon Elves and I think they could work out the way I've mixed and matched their features now.

If you just want an overview on the History of the Realms, then there's a book even for that: "The Grand History of the Realms" provides you with the earliest history of the Forgotten Realms, over the creator races, with maps of that time and images of structures and creatures, some contemporary writings of those creatures, and up to the beginnigns of 4e story.

Uh! And then theres "Ed Greenwood Presents Elminster's Forgotten Realms" with a bit of insight into the ways of living, by Word of God.

A book I can't recommend enough if you're interested in building a familiarity with Indian/Chinese/Japanese type gaming is Oriental Adventures from 3rd edition era Dungeons & Dragons.

https://www.amazon.com/Oriental-Adventures-Dungeons-Dragons-Supplement/dp/0786920157

or the PDF version--

http://www.dmsguild.com/product/23426/Oriental-Adventures-3e?term=Oriental&test_epoch=0&it=1

They made a free setting called Mahasarpa as an Indian, Southeast Asian flavored realm, as well as Rokugan (in the book) the d20 version of the Alderac game setting is a great samurai era Sengoku Jidai-ish Japanese setting, if you're looking for models.

Sounds like a good idea, like everyone has said already.

If you can, get yourself a copy of the Draconomicon for getting more details on how the dragons are and live and what not, I can't vouch for other versions, but the 3.5 Draconomicon is written in a very neat way like an explorer/naturalist writing their observations in a journal.

Many of the 5e modules have sections about running them in other D&D settings, so they are easily adaptable.

The Sword Coast Adventurer's Guide is the campaign setting book for the FR, thus far. Tho I also recommend Dungeonology by Matt Forbeck. It's a pithy little FR campaign primer and has THE best Sword Coast map produced in 5e, to date.

https://www.amazon.com/gp/aw/d/0786965800/ref=pd_aw_fbt_14_img_3/167-2967996-5756223?ie=UTF8&psc=1&refRID=2HSMNV0WGXZWD01G04RC

https://www.amazon.com/Dungeonology-Ologies-Matt-Forbeck/dp/0763693537


My favorite out-of-a-can campaign setting is actually Mystara. After I finish my next couple FR games, I'm gonna run a 5e Mystara campaign.

You can use any campaign setting book from any edition in 5e. You're just using the flavor text to tease out the world. Don't worry about the edition mechanics.

When I started making my homebrew setting back in the late 90's I used Richard Baker's World Builders Guidebook to help me design everything about the world. This book is phenomenal and will help you lay out things that you might not be thinking about, like size of the globe and weather patterns. Yes, the best advice is that you can follow is to remember that you do not need to complete the entire world when you start to design it, but from my perspective it helps to have a skeleton framework to build on when you do start out on that world.

Wizards put out a much, much better 5E screen than the "original":
https://www.amazon.com/Dungeon-Masters-Screen-Reincarnated-Wizards/dp/078696619X/ref=sr_1_1?ie=UTF8&qid=1518401829&sr=8-1&keywords=dungeon+masters+screen&dpID=516h6TMiJhL&preST=_SX258_BO1,204,203,200_QL70_&dpSrc=srch

This is the one I use now. It's actually useful.

Both books are on Amazon as Kindle and Paperbacks.

https://www.amazon.co.uk/Priest-Ratcatchers-Book-Fantasy-Hardboiled/dp/0615512151/ref=sr_1_1?s=instant-video&ie=UTF8&qid=1500495641&sr=8-1&keywords=Matt+Colville

https://www.amazon.co.uk/d/cka/Thief-Ratcatchers-Two-Fantasy-Hardboiled-Matthew-Colville/0692203214/ref=sr_1_2?s=instant-video&ie=UTF8&qid=1500495641&sr=8-2&keywords=Matt+Colville

If you are considering unpainted at all then I recommend you look at the official board games. I will spell these horibly wrong:

• wrath of ashardalon
  
• ravenloft
  
• drizt
  
• elemental evil
  
  I paid $65 CDN for ashardalon. you get 42 or so unpainted minis per board game, 5 of which are heroes. So if you bought all of them you get 20 heroes and 140 or so monsters. For reference a full brick is $280 for me, so this is way cheaper.
  
  Edit: link
  
  https://www.amazon.com/gp/aw/d/0786955708/ref=pd_aw_lpo_21_bs_img_1?ie=UTF8&psc=1&refRID=P0B2CESWQAES8CEWZBTS

If you want to do Forgotten Realms and only have $75 to spend:


5e Starter Set $13.07

https://www.amazon.com/Dungeons-Dragons-Starter-Set-Roleplaying/dp/0786965592/ref=sr_1_1?ie=UTF8&qid=1488305292&sr=8-1&keywords=5e+starter+set


Storm King's Thunder $31.42

https://www.amazon.com/Storm-Kings-Thunder-Dungeons-Dragons/dp/0786966009/ref=sr_1_1?ie=UTF8&qid=1488305205&sr=8-1&keywords=storm+king%27s+thunder

Out of the Abyss $27.17

https://www.amazon.com/Out-Abyss-D-Accessory/dp/0786965819/ref=sr_1_1?ie=UTF8&qid=1488305481&sr=8-1&keywords=out+of+the+abyss


That gets you a pretty fun sandbox from levels 1 to 5, an epic sprawling set your own pace sandbox across the Savage Frontier, and an alternative hook into the Underdark. Two to three solid years of adventuring.

You also get a ton of information on the Savage Frontier and The Underdark.

You get three different Level 1 to 5 scenarios, two different 5 to 10, then one level 10 through 15.
Lots of replay value. And you can fight a dragon, fight giants, and fight demons.