For some time I have been planning to build a ROV. Mostly just for fun, but also to do some exploration in the sea around where I live.
My design will be mostly based on the "Sea Fox" design from Homebuilt ROV.
The forums on that site has been a well of ideas and knowledge for this project.
My design will be made with as many of the shelf and 3D printed parts as possible as I live in a small apartment with only hand tools accessible. So my living room will mostly be my workshop during the construction process.

Some key features I will include are:

PVC Frame
Fishing net floats for buoyancy.
Adruino/Rasberry PI/Beaglebone for controll.
Brushless thrusters with 3D printed props and mounts.
Simple USB or IP camera for live video feed and navigation.
Gripper (manipulator) with tilt and rotate function.
On board LiPO batterypack.
Battery, electronics, camera and lights will be in separate compartments.
Oil filled and pressure compensated where possible.
Target operating depth of 100m.
Controlled from a laptop, using a wireless Xbox360 controller.

There is still some issues to be addressed and decisions to be made.
Progress will be slow and sporadic at times.