I do have the 720sw. I usually shoot with underwater macro and add the supermacro, without changing whitebalance. I find that with macro mode you have to be at least 1' to 1.5' away to get a decent focus. With supermacro mode you can be less than a foot away up to maybe 6". I'm usually shooting top down.
When the camera won't focus, but I can see it in clear focus during the prefocus (holding down the button) changing to unfocused, I'll just continually press and hold the button until it focuses right (may take 20 or more tries). The trick is once it focuses the way you want it, take the picture, but keep your finger down like when your focusing, and you can continue to take pictures at that focus length, without having to get it focused again.
There are a few different ways to have the camera focus, such as spot focus and such in the menu, but I don't know what I did with the manual, so I haven't played much with those settings, nor understood them enough to make it work for me.
These shots took me forever to get in focus. This is probably as close as you can get. With supermacro there's no flash allowed, but I like the way the watermelon turned out, looks almost like natural light.
This last shot was in macro mode. For me, which mode you use all depends on how big your tank is, and where the subject is in relation to the camera, and your arm's reach. Sometimes it's hard not to block the light when your right on top of it, trying to look through the viewfinder, which makes for some weird positioning of your body, hands, and camera. And Try not to pull a cramp or burn yourself on the halides
