I'm glad to see others posting about using the S400 underwater. There aren't many on this board that do. I think you would get great benefit out of getting an external strobe at some time. It really helps.
I really like the clown fish picture. I hope to get to a location in the next year to get some shots like that.
After reading some tips, I set the camera on manual and usually ISO 50 or 100. However I have a strobe that helps out even though I had issues of the strobe not firing (user error). Also I set the white balance to "cloudy" unless I'm shallow and it is very bright.
I also get 2 dives out of 1 charged battery assuming I don't leave the LCD on the entire time.
One of my biggest complaints is the "time to next shot". The camera isn't very quick about recharging its flash & being ready to shoot again. The external strobe recharges quick but there were times I turned the flash off to try and get a quicker 2nd/3rd shot. (Most of my Manta pictures were without flash so I could take more.. Not sure this was the smart thing to do.)
Also, sometimes it is hard to tell on the little LCD screen if the subject is really in focus or not. I use the spot focus mode.
I think the setup is easy to use and I am heading to Cozumel in a couple weeks to try it out again. I am going to work on making sure I use the strobe correctly this time.
Gulf of Mexico (Flower gardens) shots
http://www.photo.net/photodb/folder?folder_id=321273
Isla Mujeres Mexico shots (First time using the camera)
http://www.photo.net/photodb/folder?folder_id=317259
Other generic underwater (Not s400)
http://www.photo.net/photodb/folder?folder_id=223653