“There are some who say you cannot get sea sick under the waves. They’re wrong.” - Corin Hayes
Clearly for a civilisation that exists purely under the ocean waves, transport is going to be an issue. Submarines have been around for hundreds of years, and there have been many improvements and enhancements to technology over that time. However, there are certain laws of physics and nature that cannot be ignored (GMA - though as GM you might want to ignore them a little).
Sensing the environment:
All submarines sense their surroundings using sound and magnetism.
Sonar is still a thing, and a very important thing at that. Onboard computers filter the oceans soundscape and project it onto screens for the pilot to interpret. A trained SONAR operator can read more in those images than a novice.
Passive SONAR - microphones dotted across the submarines hull, and sometimes towed far behind on long cables to avoid the interference of the propellers, pick up sound, vibration, groan and squeak from the ocean. It is worth remembering that oceans are not quiet places - biologicals (whales, fish, crustaceans), technologicals (cities, factories, Boxes, mines), geologicals (tectonic shifts, volcanoes, landslides) and vehicles (submarines, Fish-Suits (”you’ll never hear me coming” - Corin Hayes), and other moving vehicles).
Active SONAR - this is the PING sent out by the SONAR operator to determine the exact position of an object. It gives away the position of the PINGing sub, but returns a lot of detail to the SONAR operator. The bigger the object, the easier it is to find. Torpedoes and other munitions need a PING to lock on, and while many carry their own SONAR and Magnetic homing systems they are not fool-proof (”the sound of your own torpedo locking onto you is trouser filling scary” - Corin Hayes).
Magnetics - Everything produces a magnetic field of some description, and some things alter the flow of the earths. The Magnetic Sensors detect these changes in flows and can be useful at short range. Some munitions switch to this tracking at very short range to avoid loss of SONAR lock due to countermeasures.
Esoteric tracking - Under the ocean, in the dark depths where light does not reach, civilisation has come up with some stranger, more specialised sensing methods, many of which are in the testing phase of development (”that means they don’t work,” - Corin Hayes). Gravimetric, Electrical, Chemical, Hydroflow (changes in flow of water, eddy currents for instance) and many others are all in development. (GMA - allow some to work in special circumstances, but be careful not to stray too far or make them overpowered. They key, for me, is that despite the space afforded by the ocean there really isn’t much to see, it is claustrophobic even in the open ocean.)
In pure gameplay terms Submarines can be divided up by class and threat level from 1 to 10 (each with the normal rule of 3 attached as per the cypher system).
Level 1 Worker Bee / Pleasure Sub
Tiny 1 person submarines with very little (no armour) and little (nothing) in the way of offensive armaments. They are quick and have some defensive systems.
Level 2 Personal / Passenger Submarines (min Bridge crew 3)
You’ll see these all over the ocean. The rich have submarines which speed them from place to place in comfort and the corporations operate large vessels to take mere mortals from city to city. Generally they have limited weaponry and a stronger defensive system.
In war, and close to the cities, personal submarines of this class are often kitted out with sensors and weapons to act as torpedo boats. However, they carry very few weapons and must return to a large submarine or city to rearm. These can be crewed by just 1 or 2 people.
Level 3 Corvette / Transport Submarines (min Bridge crew 4)
Corvettes are small, fast, and pack a punch that belies their size. Not blessed with a lot of weapon storage, they prefer short battles where they can hit, run, and then hide. Often fitted for the quietest travel possible, or speed to patrol city zones and transport lanes.
Transport Submarines are big, slow and full of cargo. They can defend themselves, make no mistake, and many a pirate has come to a sticky end tangling with a well crewed vessel. Occasionally the Navy has been known to set one of these up as bait; a crew of angry marines aboard, a few light subs close by, and a cargo hold which might well disgorge a few torpedo subs to secure the area.
Level 4 frigates (Min Bridge Crew 6)
The most common warship sported by the navies across the world. Frigates are a good mix of speed, offense and defense, excelling at none. Best described as competent.
Level 5/6 Destroyers (Min Bridge Crew 6)
Pure war machines which favour speed and weaponry over subtlety.
Level 7 Cruisers (Heavy Cruiser 8) (Min Bridge Crew 8)
These are the submarines that most Corporations use to coordinate attacks during times of war. Large, well armed and well defended a Cruiser is a formidable submarine with top of the line sensors and AI.
Level 8/9 Battleship
These are armed for war and nothing else. Expensive to build and maintain, they are falling out of fashion, but you can bet in times of war each Corporation suddenly finds a few they had misplaced.
Level 10 Dreadnoughts
The largest of all military submarines with more armour and defense systems than any other these are hard to damage and destroy. Sporting the top of the line armaments and sensor systems and with a crew of hundreds, Dreadnoughts rule the oceans. However, they are expensive to build, maintain, and are few and far between.