I always enjoyed the U-series of modules. This contains some serious (and series) spoilers.
For more reviews, please see my Reviews page.
For my reviews of the previous modules:
U1 The Sinister Secret of Saltmarsh
U2 Danger at Dunwater
U3 The Final Enemy
by David J. Browne with Don Turnbull
48 pages and a 3-fold cover with maps and illustrations
U3 The Final Enemy is the third and final adventure in the U1-U3 trilogy of modules.
Once the enemy has been identified, the PCs are sent out - not to destroy the foe, but to scout them out. This is a tough job as there are hundreds of sahuagin in a largely-underwater fortess. The PCs may have maps (if they handled U2 properly and didn't exterminate potential allies), NPC allies (a sea elf, lizard men, humans from town, etc.), and may be able to recruit more during this adventure if they play it right.
The module consists of a wilderness portion (either sea or land travel to the dungeon) and the dungeon itself. Kudos to a level 3-5 adventure for having a potential dragon encounter, as well! The plot explanations are, like in U2, a bit over-explained. What could have been done once in one section is done twice. But it's not as bad as in U2, and some of the exposition is really needed to make it clear how to run the adventure and what's expected of the GM and the players alike.
The dungeon is, like I said, largely underwater. This has all sorts of consequences. Magic items and spells work differently. Combat is restricted. The foes are all water-native and on their home turf. The adventure takes full advantage of this - there are sharks, fish, seaweed curtains, sahuagin with nets and tridents, and so on. It doesn't just feel like a dungeon reskinned to be underwater, but like a proper underwater adventure.
The PCs have a mission of determining the contents of the fortress, and the module spells out to them in in-game terms what's expected and lists for the GM what should count to satisfy all of parts of their scouting mission.
Another thing I like about this adventure is it continues something in the series - people make mistakes. The Sahuagin made one storing looted objects from previous intruders on the dry level, which allows a group to get a few more underwater-centric magical items to boost their chances of completing the mission.
I only ran U3 once. It was as a solo adventure for my cousin's thief character. His mission was the one from the module - scout the sahuagin lair. He managed this quite well, and pulled off a secondary mission and assassinated the sahuagin baron with a backstab and a desperate escape. I can't recall his level and his equipment, but it was pretty high - probably in the high single digits and well equipped. Finding the underwater gear helped immensely.
I'd love to have run it for a smart and cagey group.
How is it for GURPS?
Given GURPS Magic and Dungeon Fantasy, this would be fine for GURPS. The masses of foes are less of an issue because of the mission and GURPS mages (and DF druids and wizards) are far better equipped to get people ready for underwater exploration. You wouldn't need to hand out special magic items, just make sure the PCs knew the right spells.
Overall: An excellent peak for the plot that starts in U1. Special credit has to be given for the nautical and underwater nature of the adventure, and for being a military scouting mission instead of "sack the dungeon." Excellent adventure and well worth the read.