I'll repeat what MarkHerm said. Photography is an advanced diving skill, not something that most new divers can do safely or easily.
The problem with fixing white balance in post-processing is that most cameras don't allow you to set the ISO to one value for red pixels and to another value for blue or green pixels. Water filters out most red light in a couple feet, so an exposure for proper reds is going to completely overexpose the greens and blues, while a proper exposure for blue is going to underexpose the reds. The E-M5 has a great dynamic range for such a small sensor, but most natural light seen when diving has hundreds of times more blue light than red light, and the E-M5 can't compensate for that (no camera can, to my knowledge).
Unfortunately, the human brain is wired to respond to reds, and most of the best underwater photos have red in them. Needing red in photos and not having much red light underwater is one of the big challenges of underwater photography. There are four main ways of dealing with this problem:
1. Bring your own light in the form of underwater strobes. Inon and Ikelite are two popular brands. Alex Mustard has done some work which, I believe, shows that Inons are slightly better for cold water and Ikelites are slightly better for warm water, but both are excellent choices. The main drawback to using strobes is the short range of strobes underwater, but since the water column degrades image quality, you generally need to get close to your subject anyway for a good photo, so this isn't much of a drawback.
2. Put a red filter on your camera. This cuts down on the green and blue light, letting proportionally more red light in, at the cost of reducing the total amount of light, which means higher ISOs (with more noise), slower shutter speeds (with more motion blur) or wider apertures (loss of sharpness and DOF) are required. Since this balances the red, green, and blue channels of the camera before the signals go the A2D converter, it gives pretty good quality as long as the photo is properly exposed to start with.
3. Using the white balance option on your camera. Some people will bring a white slate or wear white fins, and take a photo of the white reference underwater to set the white of the camera. This tends to cause problems with the red in photos. Think of a camera that normally assigns a value of 0 to 999 for the redness of every pixel. Now imagine that all the reds are 0 to 9, so the camera multiplies each red value by 100 to get the full range, but it ends up only having 10 shades of red instead of 1,000, so the reds end up looking really weird and banded, not smoothly varied like the other colors that use the full of numbers from 0 to 999.
4. White balance in post-processing. This is about the same as #3, with the same drawbacks. It helps to shoot RAW in order to do this, since JPEGs lose some of the image information.
In practice, most photographers use a combination of #1 and #4. Many photographers have dual strobe setups, with two strobes on arms in order to get the distribution of light they want.
I am not going to comment on the questions as others may be more knowledgable to answer. Still one suggestion from my perspective: If you are not even certified yet, it's IMO a good idea NOT to bring a heavy, expensive camera setup for your fist dives but to concentrate on developing the knowledge and skills required to savely enjoy the new enviroment without harming yourself or the enviroment. I often see newly certified divers with big cameras, who can barley hold position in the water column (which is perfectly normal for beginners), who constantly crash into the corals in pursuit of the one great shot. It's aweful!
Of course, it is you who decides on this...
Thanks all for the suggestions and comments. And I appreciate the blog and forums as a resource and will continue to research.