Poseidon, 2203. After years of tension between the GEO, Incorporates and Natives, it appears that full-out civil war is about to break out on Poseidon. The Sierra Nueva War has dragged on for close to a decade in one of the bloodiest insurgencies since Earth dragged itself out of the collapse. Hanover Industries has succeeded in cutting its ties to Earth, earning the distinction of being the first Incorporate to be attain independence on Poseidon - a transition that was far from incident free - and is now turning its attention toward Poseidon becoming independent. GEO fears that if the instability on the water world worsens, there won't be any way to restore it without massive loss of life. Ironically, the increase of military operations on Poseidon has made it more and more difficult to dial down hostilities. Any flare up has the potential into turning into a pitched battle, every situation one that could spiral out of control.
Example Deep Angel sub fighter
Enter the UNSSV Triton, the largest submersible warship on Poseidon, as well as the largest ever constructed off Earth. Her sister ships on Earth, constructed in the late 2180s, are slightly larger but the Triton incorporates several design improvements. Her keel was laid down in 2198, and she took nearly 3 years to build. She displaces over 325,000 tons, is over 400 meters long and has a crew of nearly 3000. Powered by fusion reactors and propelled by massive engines, she is capable of supercavitation and can reach any location on the planet in under 12 hours at top speed. While other smaller vessels have been built as sub-fighter carriers, the Triton's combat wings are triple the size of the next largest ship, standing at nearly 75 air and watercraft. Two of her squadrons - some 40 craft total - are cutting edge super cavitating subfighters capable of atmospheric flight. She has state-of-the art facilities for her cetacean and orca contingent, including advanced hard-shell combat frames. Their human Marine counterparts likewise have cutting edge combat exoskeletons. Overall, the Triton is capable of projecting force anywhere on Poseidon. Her first deployment almost six months ago was greeted with open gratitude by the Incorporates, but trepidation behind closed doors. The burgeoning revolution strongly denounced it, and her deployment to Sierra Nueva sparked some of the worst Native attacks of the conflict.
To make matters worse, the Aborigines are increasing their activities. They have finally begun to understand the human motivations at play. Their conclusion is a Poseidon free from colonial Earth, inhabited by humans who believe in Poseidon's freedom, is the only way to minimize damage to Poseidon's ecosystem. To accomplish this, they need to engineer an alliance between factions that do not trust one another (and may be actively hostile), but who all want Earth's influence minimized or eliminated. The Aborigines also understand that the Triton is both the single largest threat to a free Poseidon, and the key to giving the nascent rebellion the ability to withstand a prolonged civil war.
I originally envisioned this campaign while watching the show Last Resort. The idea of the captain of a very powerful submarine going rogue struck me as being an interesting scenario for Blue Planet. Obviously the situation would be very different, as well as the potential outcomes. On Poseidon, over time the captain of the Triton would begin to sympathize with the rebellion, leading to the ultimate decision to not follow orders and switch sides. This action would tip the balance of power on Poseidon toward the would-be revolutionaries, and might actually get the rebellion into full swing.
The Triton is directly inspired by the setting for Deep Angel, which was either a video game or a movie or a property looking for one or both. Their websites are long gone from the Internet, but this site blatantly reproduces a lot of information for whatever they are using it for. Apparently there is some kind of cross-pollination between the folks behind Deep Angel and the SeaQuest fan communities, as evidenced from this blog post. It shouldn't be surprising that for any military-based game on Poseidon that the numerous SeaQuest fan sites and projects out there would be a great inspiration for vessels (something that are missing from existing Blue Planet books).
Of course, the Triton has a much larger crew than the Ohio does in Last Resort. That means it's a lot harder to take the ship and head for the nearest tropical island because the captain doesn't want to follow orders any more. Heavy automation on the ship can alleviate some of that - it's possible to let some of the crew choose to walk. If there were a fifth column within GEO that had actively worked to get members placed in key positions on the ship, that might also help explain how the ship was taken. Aboriginal influence could also explain sudden changes of heart. It's a lot more subtle than most examples of Aboriginal experiments on humans, but that would be a result of the Aborigines finally "getting" humans. The sheer cost and size of the vessel - as well as any other assets it may have - would make the GEO hesitant to just go all Thor's Hammer on the ship from orbit, and she would be well enough armed to make a standoff with other GEO forces unlikely (at least in the short term). The fact that she's faster and can dive much deeper than pretty much everything else is just the cherry on top.
The PCs would be crew members on the Triton, having to deal with both the stresses of combat operations and the decisions put upon them. When the ship goes rogue (either by the captain or mutiny), do they follow? Do they leave? Do they try to stop him? More importantly, once the Triton does throw in with the rebellion is she going to be what they need to avoid a crushing defeat or, like the Ohio in Last Resort, is she going to wind up a destroyed wreck at the bottom of the ocean?