I’d avoid putting your Nikon in a case. It won’t do much to improve your photos until you reach the limits of what you are presently using. You have 2 choices to improve the lighting and color of your underwater photos.
First choice is to turn off the strobes and adjust your white balance for the water and depth you are diving in. Once adjusted you can make the colors stand out a bit more with blue or green water filters.
Next choice is to turn on the strobes and light up your subject. Just because you have an external strobe doesn’t mean you’ll have enough light. Not all strobes are created equal and 2 is better than 1. The light at depth is filtered very fast and you’ll be lucky to get 1/3 the range underwater that you could shoot in air. You could change the ISO your camera uses to compensate but only to the point that it starts to show artifacts.
The best thing to do is get in close as possible for all your shots. If your camera/housing can be adapted to use a wide angle lens this can help.
Don’t even consider your DSLR until you can get reliable and repeatable results from your point and shoot in most cases a high end P&S can take excellent shots and the DSLR just adds unnecessary complexity to shooting underwater. A high quality photo editing suite can makes some shots really stand out and is generally worth investing your time into learning to use.
Please feel free to browse my photos they were all taken with P&S cameras and edited using Adobe Photoshop some used natural light and filters while others were taken with external strobes.