This adventure is set on Chalchiutlicue in the Sindal subsector, but could work fine on any planet with substantual amounts of ocean. Basically a visit to the underwater city of Calixcuel does not go quite to plan, the party gets caught up in a series of deadly events and have the opportunity to save the day. Whilst they might be there in the first place for reasons of their own, a good one is supplied to get them there. It's a fairly open adventure with a lot left to the Referee to develop either in advance or as the adventure proceeds, depending on your style of play.
As background, there's a fair bit of information about living and working underwater, Traveller-style. This includes a discussion of pressure at depth - I'm a SCUBA diver so it's pretty straightforward, but if you aren't familiar with the concepts the explanation is clear enough for gaming purposes. Suffice to say that spaceships and vac suits are designed to keep standard atmospheres IN, not the vastly increased pressure of deep water OUT. There's also background on the Sindal subsector and Chalchiutlicue in particular. It's a water world, and not a very rich one. Most people are more interested in survival than much else, and the world is seriously overcrowded.
The adventure begins with the party in the planet's Downport (a plan for which is provided). Here, they are invited to meet with a government official who has a job proposition - they need someone good with starship powerplants to sort out a second-hand starship reactor that they've acquired and want to use to power an undersea city. From there, they travel to Calixcuel on a submarine to do the job, a trip that takes about eight days. Why they are even on Chalchiutlicue at all is left to you, however.
Once they reach Calixcuel they are greeted with a reception and a guided tour. An overall diagram, but no detailed plans, of the underwater city are provided. It's basically a tall spike resting on the ocean floor. Scarcely have they settled into their quarters, however, than disaster strikes. As 'visiting experts' their assistance will be welcome, of course... and from here on in it is very much up to the party to decide what, if anything, they can do. There are suggestions and descriptions of various parts of the city to help you respond to their choices - read them thoroughly in advance and be ready to go with the flow.
All is very open-ended, the party may choose to attempt to escape or there is a chance that, with the right choices, they'll be able to save the entire city. It makes for a 'disaster movie' sort of adventure and ought to be played that way. It's an episode that will be remembered for a long time to come...