There is a huge amount of dive video out there and descriptions of various add ons and people's set ups, but less description of what you get in the retail box and the basic options.
They are packaged in a few different flavors based on the mounting options that come in the box, but the normal housing comes with four different backs. Two are water tight, one each with and without either the LCD bacpac, wifi bacpac or the extra battery bapac, and one each with an open vented back (for better sound quality in dry environments with wind less than 100 mph) for both the bacpac and non bacpac configurations.
With the sealed backs they are good to go diving - but for another $50 you can get another flat port housing that's designed for underwater use. You get less vignetting and better video quality with it. And it can use any of the other backs that came with the original housing.
The LCD bacpac is nice for taking hand held video and for confirming that a mounted camera is pointing in the right direction. It also lets you see exactly what you are getting in the various resolutions and fields of view that the camera is capable of, and you can play back your video on-site. IIRC cost is about $70.
The LCD bacpac is a bit of a battery hog if you leave the LCD display running, so don't plan on more than maybe 70-90 minutes of video with it, but you can turn it off and on as needed. Otherwise 2 to 2.5 hours of battery life seems to be the norm on mine so far.
The battery bacpac is available as well to increase battery life, but unless you do really long dives, I don't see the need.
There is also a wifi bacpac that allows streaming video but again I don't see a real need for the average diver.
You'll also need an SD card as it does not come with the camera. It will accept up to 32 GB so I recommend just going big and then not worrying about space in higher video resolutions. Plan on $30-$40 at the local electronics store for that.
It comes with a USB cord to download to and charge off of your computer, and that works just fine. But eventually you'll probably want to add a plug in charger. Go Pro sells one, but I found one at the local CVS capable of 1 watt or 2 watt charge rates for $15 - about a third the cost of the Go Pro charger. (You'll want the 1 watt setting as that conforms to their "fast" charger.)
I also got a 1 watt 12V usb charger that plugs into what used to be called cigarette lighters. It was about $9.
For diving you will also want the desiccant pack for it to eliminate any fogging in cold water. They come 4 to a pack and a pair of them work for about 4 dives and then you can reactivate them in an oven. They are in a heavy cardstock format rather than a bag of silica beads due to the space limitations on the sides of the housing. It also comes with a humidity indicator, but that does not work well with the LCD back since it blocks your view of the LCD display.
There are a few different companies that sell color correction filters for the Go Pro including a flip up model. I am personally kinda "meh" on the idea as I tend to use a light anyway, and there is some color correction capability in most video editing software, although not generally as much as in Photoshop for still digital photography. In any event, I prefer to shoot without color correction filters and then correct it in the post processing phase.
Finally, you may want another battery and they are only $19 direct from Go Pro.
In short, it's ready to dive out of the box, but there are accessories you'll want to add to improve the convenience or capability of the camera. So the camera itself is well priced as a bit of a loss leader but the accessories are not over priced and are for the most part a good value for what you get. Still, by the time you are done, your $249 camera will probably be a $400 camera.
-----
In terms of performance, it has very good low light capability, and the low light capability is better in the wide field of views than the narrow field of view. That's a good news/bad news thing as it means you need a very wide beam on your light, or a pair of them to light the full field of view, but you can choose the field of view that works best on your dive, and generally I'd recommend using the wide field in high def unless you have a very specific reason not to.
