This is the first draft of the ROV's interior. Though the current production draft fits a bit tighter with the camera tilt servo and faceplate (which protects the raspberry pi from water drippings when you remove the dome), I think it gives you a pretty good idea of the hardware layout. You have a LiPo battery compartment down the center of the 3D printed "cradle" (out of view), which kind of acts like the interior's core. This fits a 2200mah 3S lipo, but let us know if you have any ideas on how to get a bigger battery in there!
Then we have a Raspberry Pi 3 Model B mounted forward of the LiPo compartment. This is positioned with the GPIO pins facing the rear dome. That makes it pretty difficult to accidentally unplug anything while you're getting set up for a dive.
On either side of the dome, we have our dual channel motor drivers. These communicate with the RPi via PWM from the GPIO pins. To make any changes to the GPIO setup, or add hardware, just unscrew the right-hand motor driver.