Ok, First things first, learn to seal the case correctly. You need to use the lubricant provided on the O rings, but don't get nuts with it, a thin coating only. Also make sure the O rings are clean and free of sand and debris of any kind. A small thread from a towel you use to dry the case is enough to cause a leak if it falls across the O rings boundry. Second, Although this is going to be a night dive, it is a good rule of thumb to keep the camera housings temp as close to water temp as possible. This will prevent fogging the lens when you enter the water.
Do you have an external strobe or just the one on the camera? If you have an external, depending on what housing you are using you may need to use the strobe in pre-flash to get it to time correctly. With your internal strobe only you may have trouble illuminating large subjects so you may just get what is in the beam of your dive lights. If you know that, great, you can compensate and or you can use it to your advantage by using the dive lights illumination to help you with your shot! You might bring two dive lights, one with a difuser lens in it and one without. This give you a wider illumination but not as bright and then one that is brighter that might draw attention to a particualar area of the image.
It is not as powerful as a strobe but alot cheeper!
DM