This is the Zock. People often mock it for its odd looks and bulk and supposed lack of charm. However, a rudimentary understanding of aerodynamics and such explain its features immediately; it is meant to swim lying prone.
Aquatic Mobile Suits are not intended to swim standing up, they're meant to swim like a human or seal or anything else that moves through water does. They're like submarines, or more like torpedoes; usually with shapes that are intended to give them the advantage in movement through the water.
The Zock is incredibly specialized in this sense. Its entirely symmetrical body gives it ease of piloting, weapons on all sides for attacks on both other submerged machines and surface shipping as well as general surface destruction, giant movement systems under its skirt to allow it to move Dom style and kick ass on land, and a monoeye conveniently mounted for all of this as well, its rail lending itself to full coverage of the Zock's whole body. In the anime it only briefly got to appear, but "We Are Federation Hooligans" gave it a pretty good run.
The Acguy, Zock, and Gogg all follow similar principles in different ways. The Acguy has full monoeye traverse and head vulcans, but is more humanoid because it is meant for more emphasized land use. The Zock and Gogg are similar, with the Zock having both the monoeye traverse and head weapons, and the Gogg with head equipment (freeze gel), retractable limbs for a good swim, and most importantly making use of water for its reactors and in turn a pair of Mega Particle cannon, to be inherited by the Hygogg.
Suits like the Agguguy, Juagg, Juaggu, Agg, and marine adaptations of land MS (e.g. post OYW Zaku Mariner) have some of these traits but not all as they are meant for different roles entirely. They aren't meant to be devoted swimmers, but more like human divers and so are less streamlined, though in the Zaku Mariner's case it still had expanded monoeye traverse and weapons built for use while swimming.
