Hi @Riley_Regular, welcome to the forum 
The Navigator is a peripheral board for a Raspberry Pi, and is currently only supported with the Raspberry Pi 4, so you’ll at least need one of those (which can be bought together with the Navigator).
A Raspberry Pi can communicate over ethernet, so a minimal setup would just be Navigator + RPi + some form of ethernet cable as a tether. That said, ethernet has quite a short range and uses several conductors, so we typically recommend using converter modules (like our Fathom-X) at each end, so you can send the ethernet signal through longer cables and through a single twisted pair of conductors (like in our slim tether).
If you want things to be a bit cleaner at the top side and/or if you have a USB port but no ethernet jack then you might want our FXTI instead of a bare Fathom-X board.
Vehicles that require higher bandwidth communications can use fiber-optic converters instead, with a fiber optic tether, although they can be pricey and require some extra care to connect and to handle during operation.
Our typical ROV setup has BlueOS running on the Raspberry Pi, with the ArduSub autopilot firmware running the Navigator side of things (this comes built into BlueOS), in which case using a game controller with QGroundControl or Cockpit are your best options for communicating with and controlling your vehicle.
I’m in the process of making some diagrams about this kind of thing for our documentation, but here’s an initial sketch of the typical stack:
If you’re wanting to write your own low-level code to directly read the Navigator’s sensor values and control the motors individually then it’s possible to use our Navigator libraries instead of running the ArduSub firmware, in which case you’ll need to also write the code that interfaces with the top side, and you can use whatever topside control software you choose to make it compatible with.