Manual white balance is an absolute must. Trust me! Either carry a white slate or paint your fins.
If you're going to get into uw video, then be prepared to spend big time. My camera (Panasonic 3CCD anamorphic widescreen) cost about €1200. The housing & lights added about €4K to the bill...
You can get cheap or you can get good. The cheap setups will end up frustrating you as you don't get the control you need for trickier shots. Remember that current DV technology doesn't allow the flexibility that RAW stills do. You need to get it right & tight in camera.
It only becomes worthwhile when you get that perfectly captured magic sequence, and then it all begins to make sense (but only aesthetically, never financially
)