Earlier this year I wrote a blogpost called Getting Flash this video blog covers the same stuff at the beginning about flash exposure, but goes into greater detail. However apart from getting the flash exposure right, and controlling your strobes manually which is of course important, the second hot topic in UW flash photography is flash positioning so this video guide which is just over half an hour in length, looks at twin strobe and single strobe flash positioning. For both wide angle and macro shooting.
The video is around 35 mins long. I hope you find it useful. If you want to view it full screen, click on the YouTube tab in the bottom right corner of the video below to open from within YouTube, and adjust the settings to 720p HD.