My concern is that Aquali sounds too unstable and unforgiving to really sustain a Space Marine force.
From your lore as written:
The planet is composed of mostly water with several small islands with frequently join together only to break apart months later if not years.
Rapid tectonic activity like this, by itself, would mean almost no space above the water level on the planet is habitable for long without regular and recurrent terraforming. The islands wouldn't connect, they would slide into over and under each other.
The waters of this planet change temperature during each season from boiling hot to frosting cold.
This is interesting, and somewhat reminds me of Crematoria from the movie Chronicles of Riddick. This is a good point to include, and i'll explain while further down.
The predators are more than eager to feast on the populace, even the creatures the feral aqualyans must hunt to survive can slay any man even a non-armoured adeptus astartes would barely be able to slay such a beast.
Predators that require a fully armoured space marine to take down? and this is what the populace of Aquali must hunt to survive? This has a nugget of potentiall, but as written alongside heavy casualties would decimate the human populace potentially long before it was the Silent Dagger's Chapter world.
There are numerous tribes and clans of said aqualyans who must kill one another just to survive, few of the even grow to the age of 20 let alone 30+. the children and infants rarely survive to their teen years let alone adulthood. Some clans and tribes are slain to merely one man these men or women are forced to survive on their own often choosing to die fighting but some manage to barely live the rest of their lives in solitude until their inevitable deaths.
Death Worlds are already kill or be killed worlds, adding a fuedal human society to that is overkill. Tribes fighting between each other for sparse resources does make sense however.
These horrific conditions often turning these men and women into true survivors, worthy of serving the imperium of man. In the depths of these oceans are many valuable minerals which have been used by the Silent daggers to build much of their gear. The people of this world are silent hunters using the art of stealth to survive and adapt to their environment, while eager for the kill, they still understand that patience is critical in this art.
So heavily ingrained survival instincts, along with a penchant for stealth.
Right, lets makes some suggestions.
A heavily tectonic world would be almost impossible for humans to settle on. Instead, keep the seasonal ocean temperature, and use that as the cause for almost constant planetary storms like typhoons. This would create a people who are very resilient, and able to change their plans as a moments notice depending on how quickly storm cells form. These are excellent attributes for potential space marines.
Instead of the populace having to Hunt the planets' apex predators, consider that before humans settled on the world, there would have been prey for it. It would make more sense that the humans domesticated these prey animals, and so feel under constant attack by the predators. Depending on what these predators actually look and behave like, it could be written that as a child reaches their 10th birthday, they must go out and survive beyond the city/settlement walls to prove they are worthy of being part of the clans, this would also highlight potential candidates to the Space Marine recruiters. Those that fail this trail would obviously be killed, and so only strong humans would return.
You can still have the Apex predators be a threat even then, as the trail would ensure the child can survive in the wild by quickly learning and adapting to stealthily traverse the terrain.
The populace needs to be self-sustaining in order to even survive, and doubly so to be a recruiting world for a space marine chapter. Considering that neophytes are chosen from around the age of 13 to be inducted into the Chapter, a large enough portion must be declined in order for the world to be sustained.
Considering the above paragraph, those that survive their trails but are not selected to become marines would be highly valuable warriors, and would likely be trained still by the chapter to act as a planetary defence force. This would greatly improve the longevity of the populace. Keep in mind, it likely wouldn't be enough for the populace to push back against the nightmarish predators, but enough to protect themselves.
Which leads us into the clans this world likely has due to the humans originally surviving like nomads, as they would have had to keep on the move to avoid the predators. Fights, skirmishes, and outright battles would likely have occured at some point for settlements to be established. this can also feed into the different marine squads, and how they're trained, as different clans would have survived in different regions, and so would become specialised in different methods of combat, hunting, and survival.
I like the aspect of the Outcasts, but you haven't explained or explored them enough in your lore yet. A Lone Wolf which you compare them to is the sole survivor of his pack being destroyed. He is geared with the best weaponry.
An outcast on the otherhand is more likely been thrown out of their group, for some infraction or maybe just not being able to work in a team. I'd imagine they would have a "Keep what you kill" mentallity to weaponry and wargear, and along those lines likely the earliest adopters of Xeno-tech in the chapter.
Your initial trails are flawed.
The first one by your original description is basically a suicide mission. How can a teenager hope to hunt and kill one of the planets apex predators without any of the marine enhancements? This Trail would see a quick end to the chapter.
The second trail is interesting as it highlights that the chapter would have a proficiency in underwater combat. This could easily be respresented by treating any terrain feature with water as open ground, as Mabbz suggested earlier.
The third trail is understandable, hopefully by this point the aspirant would have been put through some conditioning, otherwise this trail would see a high fatality count.
The fourth trail is alright, and works with pre-established lore that even going through the process of being augmented can break an aspirant.
The Mask of Silence is interesting. It basically supersedes a couple of the organs a marine would be implanted with, probably making them redundant. The ability to commit suicide at will seems a bit extreme, and at present there seems to be very little about the chapter that would warrant it. Keep in mind if they're captured by an inquisitor, there is a very high likely hood that even death will not keep them quiet. Its flawed in that as the last implant that it can outright kill an aspirant. This shouldn't be the case, as it would quickly be removed from the augmentation list to ensure a higher recruitment survival rate.
The Battle for the Aria Subsector sounds great, and works well to introduce why you'd include Storm Troopers and Knight Errants in an army.
Now onto rules.
Shorter height and likely greater flexibility from underwater combat training would easily be represented by a +1 buff to their Initiative or Attacks, or some sort of reflex save. However, you'd have to balance it out by making them less proficient somewhere else. Usually the trend is faster hitters have lower strength, and harder hitters are slower.
As you've yet to tell us who they're taking xeno-tech from, we can't suggest much for what to do with wargear unfortunately. However, I'd suggest having a higher number of blade based weapons to work with their greater speed/agility/flexibility. guns would be best if they could be used one-handed, with exceptiong for you Far Blades whom I'd guess would have more Harpoon-themed weaponry alterations.
I'd assume your Librarian would have very aquatic/naval inspired powers, such as psychic harpoons, and summoning surrounding water to create tidal waves on land.
thats what I have to say for now, but just keep in mind its just my point of view