Expedition introduced a lot of great content in the form of new weapons, the introduction of Jet Skis, and new places/dungeons to explore. When it comes to how Expedition has been integrated into the main game, it has been done pretty well, but there is still room for it to be improved upon.
As it stands, it feels a lot like the Waterways are just another part of the Black Sea, instead of being a part of Southern Underrail's Lower Cave network. Looking at it from a new player's point of view, once the game opens up after Depot A, the player is given free reigns to explore the world as they see fit. Now, if a player just roughly follows the various quests they are given, they will be taken all over the Lower Caves, Lower Underrail, Upper Caves, and Upper Underrail, at least eventually.
That being said, a new player will not ever have a chance to explore the Waterways unless they accidentally stumble upon Ray's shop and decide to invest a lot of money into a Jet Ski without knowing whether or not such an investment will or will not pay off. And if a player instead goes the route of joining the Expedition, which I feel is much likelier to occur before a player buys a Jet Ski from Ray, they will spend a lot of time in the Black Sea before they ever have a reason to return back to the Water Ways. Eventually, once the player is much higher level and has likely already completed much of the games quests, they might start to head back to the Waterway's, and only then will they see how it connects to much of the locations they had visited previously, and will able to access the Waterways content.
The way things are designed now, the player will be far more likely to first interact with the grand Lemurian ruins before they ever get a chance to explore the Waterways. It kind of feels like a step down to go from being the first person to step foot in the ruins of an ancient, advanced civilization that was around hundreds of years ago, to just jetting around the Lower Caves and looking at some of the stuff that dots the Core Ring. Plus, accessing the Waterways at such a high level takes away from the experience, as at this point the player will have already seen much of the games most difficult enemies, and will no longer have as much fear/reverence when it comes to exploring as they did directly after Depot A.
There are also some minor points in that Hell's Gut is never mentioned once anywhere in Southern Underrail, but once you meet Ladleman in the Expedition Camp he makes it sound like it's something anyone from Southern Underrail would know about. Or the fact that South Gate Station is also connected the Lower Cave Waterways but doesn't seem to have any presence within them at all.
So, I propose giving the player an opportunity to access the Waterways once the map opens up after Depot A. The player already receives several emails which lead to quests that will lead the player to Rail Crossing and Camp Hathor. Perhaps a third email could be sent that directs the player to speak with Captain Svana or another NPC that could be added which would give the player a quest to be resolved somewhere in Chancer's Bay, which is directly south of the SGS docs. The player could be outfitted with a Junk Jet which will give them an opportunity to explore the Waterways right off the bat.
This will have the added benefit of addressing one of the main complaints people have about Jet Skis. Jet Ski's are very fun, but feel very underutilized throughout the game's massive runtime. In the Black Sea, the vast majority of your time will be spent off of your Jet Ski, exploring Lemurian ruins. You still get to spend a decent amount of your time fighting and exploring Hell's Gut on your Jet Ski, but this isn't enough, and I feel like the development team agrees with this statement as a lot of work has recently been put into adding more content to the Waterways of lower Underrail.
Giving the player a way to explore the Waterways earlier also allows for more content to be introduced to flesh out the Waterways as they still feel underdeveloped (will reference Chancer's Bay again, as I've purposefully Jet Ski'd around all of it to to see if there has been any meaningful content introduced, but I didn't see anything outside of what might've been a random encounter that got resolved off screen). SGS is considered one of the strongest factions in Southern Underrail, able to rival the large Foundry, so I think adding some SGS outposts or Jet Ski Patrols immediately surrounding the SGS docks and perhaps into Chancer's Bay could be a good way for this to make sense to the player as well as having the added benefit of integrating the Waterways into the main game even more.
Finally, I think it would be nice if some of the existing NPCs or perhaps the new ones referenced the Waterways and the Black Sea just occasionally, even before you've reached Core City. Doesn't have to mention anything about the Expedition, but it would be nice to get something related to the Hells Gut and the Waterways established early, just so it flows within the game a little better and makes Ladelman's dialogue consistent with the players experience.
Just a few suggestions I have that will hopefully be of use to the dev team. The ideas are just me spitballing, what's important are the specific critiques regarding what I feel is lacking from the game atm.