Murder on the Primewater Pleasure is a fair mystery game to run but is far from perfect. It directs you to run it right after they complete the "Salvage Operation" quest from the book, making the assumption they have successfully recovered the lost cargo and brought a small fortune to the host. It asolutely must be run at Level 4 since any lower would have the party too weak to fight the killer and any higher would give them access to spells like Revivify and Speak with Dead that could ruin the whole mystery aspect of the quest and I appreciate the creator for taking this into consideration when making this product.
There is a lot to like. The idea of a self-sailing ship is creative (with the added bonus of cutting down the list of suspects with the limited crew). The actual murder itself: how it is presented, carried out and the misdirection and careful consideration of spells and material components used is convulted enough to be interesting yet easy enough to follow along with. The statblock for the fight with the killer is pretty good (I always appreciate good movement options) and despite it being 1v4+ in the player's favour it should still prove quite challenging.
In the terms of fitting this into a larger Ghosts of Saltmarsh campaign the module presents itself as a way to expose the Scarlet Brotherhood, a shady third faction working in the shadows of Saltmarsh towards chaos and conquest. However, it also kind of kills any further impact they can have since the leader of the Brotherhood's Saltmarsh branch is meant to be the murderer and be killed/apprehended by the end, all this does is change the complication for a DM from "How do I introduce the Scarlet Brotherhood?" to "How do I still make them threatening when their most dangerous member in Saltmarsh just got taken out?" instead of solving anything. Obviously you could just make your own leader but the actual mastermind of Saltmarsh's Brotherhood is already such a good reveal it seems a shame to waste it here.
Furthermore, the adventure is built for 4th level characters but seems to be working on the assumption the PCs have no knowledge of who Anders Solmor and Gellan Primewater are despite them being very important NPCs in Saltmarsh they've almost certainly encountered before. For Anders this is easily fixed by just editing the text that is meant to be spoken during his introduction although the sudden appearance of his never-before mentioned fiancee might raise a few eyebrows, even moreso when they learn she is daughter of another important NPC (Eliander Fireborn). However, if the PCs have already worked out Gellan is connected to the smugglers whom they've encountered a few times now in the quests from the book you might have a problem justifying why he's inviting them on a cruise ship without them being immediately suspicious. Even more egregious in this reviewer's mind is that since Gellan is being framed by the real killer, it's entirely possible the PCs will fall for it. His defence when confronted by them is painfully weak and only incriminates him further as he admits to being involved with smugglers. It's literally "Do you really think I'd kill someone and draw attention to myself?" At which point I honestly think most PCs would say "Yes; you thought you'd get away with it."
Perhaps the weakest part of the book is the "Coded Letter" handout. It's meant to be a letter from a secret organisation that has gone undiscovered for years and the code they use is the simplest and easiest I've ever seen. This letter is what reveals the killer is operating under orders from the Scarlet Brotherhood, so it needs to be simple enough for them to solve whilst still on the boat so they can actually confront and catch the guy but it stings that during an adventure that is meant to unveil this grand shadowy conspiracy to the players it kind of makes them look like amateurs.
I hope it doesn't sound like I hate this module. I don't. For a "Pay What You Want" product it's really very good. The adventure is clearly very carefully balanced - as any mystery has to be - and changing too much can open up plotholes so I just feel a DM has to be gentle when trying to put this in a Ghosts of Saltmarsh campaign. I feel like it'd almost work better as a carefully presented one-shot using the Saltmarsh setting.
[3 of 5 Stars!]