You can find previous posts in this series HERE.
One cool thing about the DMs Guild is that creators often release a lot of material to support current storylines. The first three products for today would all be useful in a Saltmarsh/nautical campaign.
Daerdan's Tall Tales: Ten Seafaring Encounters from Wyatt Trull
Shorter previews of longer products are sometimes throwaways, but this is an exception. What you have here are ten encounters taken from a much longer supplement, 101 Seafaring Encounters. The author utilizes ideas drawn from 4e's skill challenges, offering some creative scenarios for a Saltmarsh/nautical campaign. I really appreciated the monster and magic item index at the back. Bonus points for using a dragon turtle.
Saltmarsh Ship Battle Maps and Sheet from Louis Porter Jr.
This is my favorite ship sheet that I have seen so far. Really clean and easy to read. Also includes some maps with the different "levels" of a ship.
Encounters on the Savage Seas III FREE from Multiple Authors
Woah! GMs are getting spoiled today! These six nautical "encounters" are more like mini-adventures. I really liked the "Gray Matter" encounter...reminded me of Creepshow 2. If you have ever seen it, you will know what I mean. Anyway, if you dig this, consider purchasing the other two in the series HERE and HERE.
The Grim Reaper-Warlock Subclass from Midnight Tower
You had me at "zombie cat familiar".
Secret Spells of Gravity Magic from Art of Karthik
I absolutely LOVED this short collection of physics-inspired spells. Pull foes into an elliptical orbit, then let em' fly with Newton's Slingshot. Warp gravity and vanish with Pound-Rebka's Slow Time. Great stuff...but that Sphere of Annihilation material component is gonna be tricky.