So I did what everybody does: I tried a cheap topside tripod I did not use anymore (Dynex DX-NW080 ~ $20) in the pool. and although it did not explode or rust instantaneously, it did not allow me to get close enough to my subject when I tried to use my macro lens (I use a Sony CX550V in a Light&Motion Stingray G2+ housing with a flippable macro lens that can be used only with the flat port).
Indeed, what is needed is something that allows you to get low and close to your subject. But sometimes, the nudibranch you want to shoot is sitting on a boulder and underwater tripods I referred to above usually won't allow you to reach that far. They are also limited to a minimum height anyway (about 6 inches for the unit discussed here).
So before I jumped the guns and bought one of the existing dedicated tripods for a lot of (potentially wasted) money, I went into a brick and mortar photo and video store and took a close look at the (topside) tripods they had on display and played with them. That's the nice thing about brick and mortar stores that you can hardly dream of doing online (I am writing that for the next generation of readers, who probably will have to look up "Brick and mortar store" to understand what I am talking about).
I actually found quite a few that could get pretty low. 11" is about as low as I could find, and I found one which was relatively affordable and looked sturdy enough: Cullman Nanomax 250. It's available online for ~$50 but I bought it where I found it (for more) considering that I had been able to try it and occupy their floor space for quite some time...
Edited by uwxplorer, 12 February 2012 - 07:43 PM.