Welcome to the forums!
I like high seas campaigns-- it's a great way to have a framework to travel from interesting place to fantastic locale; no two sessions need to be alike.
The campaign won't be so different than others you might come up with, but there are definitely some key features and questions you want to confront before you start.
First and foremost, the characters.
Are you characters already seafaring? Do they have oceanic professions and backgrounds? Do they have nautical themed 3rd level Class paths?
Or are they landlubbers? Were they pressed into sailor service? Are they seeking a new life, or fleeing their old one?
What draws them to the sea? Have they heard rumors of exotic ports rife with adventure? Are there tales of sunken treasure and cities? Do the seas hold some threat?
These are great session 0 questions for your players, be ready to prompt them with content you know from your homebrewing. A handout will help.
Second, and this is my favorite, your ship.
If you are using the ship as constant transportation from scene to scene, it will be a persistent location and home base.
What is its layout? How many crew her? What is her history? Is she a charter, an explorer, a work ship? Who is her captain, and where does she make berth?
What is the economy of its operation. How does she make venture and profit. Who owns her?
How many masts? How many yards of sail? Does it use magic propulsion? How many days provisions in her hold? How many repairs?
I'm a nautical nerd, so I love these details. Your results may vary. None of them are absolutely crucial, but all of them can be prompts for adventure hooks or complications.
This blog is actually a tremendous resource for me; the author is creating a scale model replica of the HMS Terror, and I found it invaluable to understand how ships were constructed, and how they work.
There are quite a few resources available online to help your ship come alive, steal frequently!
Third, the sea-- the most important character.
What are the seas of your world like? Where are they calm, where tempests? Is your world elliptical like ours, with arctic extremes and tropical bands?
What lives in the seas? What intelligent races? What sea creatures? What nations ply the waves? Are naval conflicts a feature of warfare? What technology? Cannon?
Certain things will be unavoidable, so be prepared:
Someone will fall off the boat. This was fatal, historically.
Enemy forces will attack. How will they board? What tactics will they use?
Storms and nautical hazards will be ever threatened-- reefs, crags, volcanos, tidal waves, icebergs, saragasso morass, even calmed seas are challenges.
Finally your setting, the spice in the soup.
What lore do sailors keep? What are their superstitions (you better believe they have them). What tales do they tell? What creatures are rumor, which real?
What of pirates? Do they plunder the waves, or are they economic operators? Do they keep a code, or are they just marauders. Have they honor or roguery.
What is the history of the seas? Are there fields of former conflict, littered with shipwrecks? Are there ruined civilizations sunken beneath the waves? What deities are prominent?
There's no shortage of books, tv and movies to draw inspiration. Again, steal like a pirate.
Pirates of Caribbean
Treasure Island
Moby Dick
Black Sails
Vikings
One Piece
Pirates of Dark Water
Master & Commander
Crossbones
20,000 Leagues under the Sea
Peter Pan
Goonies
Castle in the Sky
Princess Bride
Waterworld
Horatio Hornblower
The Terror
The Perfect Storm
Life Aquatic with Steve Zissou
Robinson Crusoe
Swiss Family Robinson
The Last Ship
Ghost Ship
I would be remiss if I didn't also include the fine podcast episodes from Dungeon Master's Block pertaining to issues nautical:
175: The Great Voyage (feat. Rich Howard)
173: RAHH Real Monster XI; Storm Giants (feat. Chris Thompson)
146: Treasure Maps (feat. Joshua Lorimer)
128: RAAH Real Monsters VIII; Kuo-toa
DM-Nastics 107: Wetter is Better
105: Are We There Yet? (feat. Will Jones)
DM-Nastics 103: Come Sail Away, Come Sail Away With Us
37: Return to the Depths (feat. Rich Howard)
26 Arr Ya Ready Kids!?!? (feat. Shawn Ellsworth)
And I'm sure there are others.
For 3rd level magic items, I'd just use the shared campaign treasure point system.
Give players 8 treasure points.
Let them pick items from DMG Magic Item Tables A, B, C or F.
Items from table A, B, or C cost 4 points.
Items from table F cost 8 points.
Good ventures!
