Jeff, this is a copy of a post I made several years ago on Wetpixel. Although more aimed at still photography, it may be applicable for you.
I've flailed my way through the tripod learning curve, so I hope my mistakes can help you.
My first question would be: Why do you need a tripod? If it is to just hold the camera in a spot for daytime (conventional) shooting, like poised to shoot remote triggered images of garden eels, the ULCS solution may be all you need.
If it's to extend your shutter speeds out past 1/15, to the relm of 1 or 2 seconds, the ULCS tripod fails to deliver. At least for me.
As to the latter, here's what works for me.
Generally, the two important words I'd use to describe an underwater tripod would be big and heavy. Simple, and cheap, would be good as well, due to the wear and tear of salt water.
Big
I use tripod legs that are roughly 72" extended, and 24" collapsed. This is the equivalent of the Manfroto 3001 legs (which I don't use, see Cheap below). This size choice is because anything smaller has too small of a footprint for stability, and, most importantly, anything shorter has me too close to the bottom to frame the image through the camera. I usually have the legs extended by one section; this seems to give decent clearance. Big is also robust enough for a heavy housing.
Heavy
If it's too light, it turns into a kite. The effect here is magnified by being in the water. Don't even bother with carbon fiber, it's too light! I actually dive with an extra 5 lbs on some webbing that I sling under the ball head. I estimate that the whole assembly - legs, ball head, housing and lead - have an underwater weight of about 10 lbs, which is barely adequate for 2 second + exposures. (note) Make sure you BC has enough reserve bouyancy to haul this around, especially if you use a wetsuit and are deeper than 80' or so. Most BCs don't...
Simple
Just plain old tripod legs are all I've needed. The geared center column is just another thing to fail and I've never needed one underwater, anyway.
I cannot reccomend a real ball head enough. It is much easier to use (especially when you are well narced) and faster. The video type pan-tilt heads are truly a PITA even for surface shooting, let alone underwater. The quick-release plates are actually kind of useful, although I'd save my money and pass on the Accra-type plates.
Cheap
The Manfroto 3001 legs are perfect. Unfortunately, even on eBay it's about $70 to pick some up - used. I found a dealer on eBay, amvona.com, that sells tripod legs that are 90% of Gitzo quality at about $20. I reccomend this model from them. As for the ball head, the Bogen 3055 is an excellent choice, as they are old enough they go for cheap, and they have a delrin ball that won't freeze up from corrosion. (happened to me) You can get one of these for about $40 on ebay with the quick-release plates. Do NOT accecpt any head smaller than this, you'll regret it, usually at the swim platform.
For me, a typical dive with a tripod consists of seeing the shot, and hovering above the bottom while extending the legs and attaching the lead. Then I'll sink slowly to the bottom, place the tripod, and frame the image. Shutter set to self-timer, squeeze off the shot and let go until I hear the shutter fire.
As I said, a smaller/cheaper tripod may be tempting, but if you're serious, just bypass the ineffective stuff and get something that will work.
Here's a photo with my tripod in use from a couple of years ago:
All the best, James
