I use a sealed high beam in PVC. It usually gets about three and a half hours of light. Rather than pulling a car battery in an innertube I use a lawn tractor battery and put it in a backpack. I find it works better for me because I gig near an oyster bar so I don't want to constantly drag the tube over them. However, it can get heavy after a long night. As for the wading depth, it depends on the situation. I don't go past my waste since the battery is strapped to my back. However, some of my best flounder have been gigged in just inches of water. This usually accompanies a rising tide and baitfish in the reeds.
Check out this site. It's just the basic start, I have made many alterations but this should help. If you have any other questions feel free to ask.
http://www.thejump.net/fishing/flounder-gigging-rig/flounder-gigging-rig.htm