The Ghosts of Saltmarsh module is a Fifth Edition re-release of several old AD&D adventures. It includes all three of the U series (The Sinister Secret of Saltmarsh, Danger at Dunwater, The Final Enemy) by TSR, and several from Dungeon magazines. In the introduction, the book is positioned as either an episodic set of adventures for a campaign taking players from level 1 up to 12, or as a collection of side quests to be inserted in any seaside setting. However, the truth of the book is that it's far more than just a collection of adventures. It also contains a detailed profile of Saltmarsh as a location, plus a rule set for shipboard combat, ship stats, and new monsters and magic items. I picked up the special edition of the book at my local game store. I have yet to run or play any of the adventures in the book, so I'm basing my impression of it exclusively on reading it from cover to cover.
While I have no experience with TSR's U series, this book isn't my first encounter with Saltmarsh. I was first acquainted with Saltmarsh in the Dungeon Master's Guide II (DMG2) for 3.5. Chapter 4 of DMG2 was titled Saltmarsh and provided a city map of the town, and profiles of its most important citizens and locations. I still have my copy of DMG2, so I did a comparison of the city maps in each book.
It seems that Saltmarsh has gotten sparser since the Sundering, although a few key locations remain. These include Sharkfin bridge, Hoolwatch Tower, the cemetery, the Carpenter's Guildhall, and a leatherworks. The leatherworks location is of particular interest, as in 3.5 Kiorn Kester was the guildmaster (and the location itself was the Leatherworkers' Guildhall), but in 5e it's a Kiorna Kester who's in charge and the location is called Kester's Leather Goods. The Kesters in both are human, so I assume Kiorna Kester is a distant relative with an eerily similar name. Kiorn Kester, according to DMG2, was in league with slavers, and the subject of a very public trial that transferred ownership of the building to the city. An adventure hook in DMG2 revealed that the guildhall's deed could yet be found, so it may be that Kiorna Kester represents the resolution to that hook.
Ultimately the point is that the Saltmarsh of 5e is a new, simplified, and fresh Saltmarsh, with little to no applicable baggage you need to worry about if you're new to the location. The main hook is this: Saltmarsh is a rotten little seaside town with smugglers, pirates, with lizardfolk and sahuagin settlements in close proximity. It was originally set in Greyhawk, but is comfortably a dual-citizen now of either Greyhawk or Forgotten Realms, or any other world you want to use it in.
Prior to Saltmarsh's publication, there was an Unearthed Arcana release entitled Of Ships and the Sea, which proposed some rules for ship combat. This got adapted and expanded, and is Appendix A in Ghosts of Saltmarsh. I didn't much care for the rules then, and I frankly don't much care for them now. Read my post on the subject for more information, but it boils down to this: combat performed as an exquisite corpse just doesn't work for me or for the groups I play with. I've got some homebrew rules for ship combat that I'm still play-testing, and it seems to satisfy my groups so far.
Just because the rules don't work for me doesn't mean the rules are bad, though. I quite like the ship stat blocks, and since combat is mostly derived from stat blocks, I think that with the right group it could be a lot of fun. However, even with the right group I feel like the act of entering naval combat would be a major shift from established RPG play style. All in all, it reads more like a wargame than an RPG, but not enough of a minigame to feel like a fun and brief break from the norm. Luckily, naval combat isn't a requirement in any of the adventures, and even if it comes up in an adventure, there's always the fallback of just having the attacking crew board the target ship for hand-to-hand combat.
The best part of Appendix A is all the stuff the Unearthed Arcana play-test material didn't include. After the naval combat rules and ship stat blocks are random encounter tables for the open sea. There are even separate tables for different tiers of play. But even better still are the islands and underwater locations provided, including ship wrecks and ancient ruins, complete with a map describing the topography of these underwater areas. If ever you wondered what to do now that your players had several Water Breathing scrolls or potions, you'll definitely find the answer in Appendix A.
In spite of having spent hours poring over the city map and the underwater locations, the real reason I bought Ghosts of Saltmarsh was for the adventures. Ghosts of Saltmarsh is a little unique, because you can treat the adventures as one-shots or side quests, or you can play them all in succession for a campaign that's only thematically linked. I like this idea for its flexibility, but also because I'm generally not a fan of big epic campaigns that last for months at a time. Whether I'm acting as DM or a player, and despite taking ample notes, an epic story played over a year or more inevitably gets so muddled that there may as well be no connection between early adventures and the finale. It makes much more sense, for me at least, to play unrelated episodes for a month or two each, allowing for the occasional callback or reference. A common but notable location like Saltmarsh makes it feel like a proper campaign, and the player characters all get to level up in satisfying ways, but there are no complicated soap opera story beats to keep track of.
The stories in Ghosts of Saltmarsh are all strong, and by using a series of vaguely connected adventures (the U series, specifically), there are just enough internal references to make them feel connected to Saltmarsh itself. Even so, there's enough exploration and travel to make adventurers feel that they're adventuring, with Saltmarsh being a reliable stronghold or safehouse, and never a prison.
Realistically, as long as you've been a Dungeon Master for a while, it's not too difficult to grab the original adventures and convert them for 5e. You adapt the skill checks to modern terminology, you use monsters from the Monster Manual, you mind your CR budget, and no one notices the difference. If you've got the DMG2, you even have plenty of city data to draw from. However, only Ghosts of Saltmarsh provides you with the premeditated structure of adventures, of the creeping threat of lizardfolk and sahuagin politics, dozens of underwater locations and adventure hooks, exciting new monsters, and a fresh new look at a dirty old location. This is what Wizards of the Coast does best: they bring the game to their ruleset so you can sit back, read up, and enjoy.