Hilary, IVO what you said I would start with a good small u/w flash like the Inon S2000. One must have a good strobe to render colors correctly u/w. Some folks use high powered flashlights to paint the picture, but they may scare off your subjecst and need to have high wattage & lumens.
Fisheye FIX has several options for adding close-up adaptor lens which you could look into.
A tray and handle to link it all up is IMHO a necessity.
As far as technique goes the hardest thing to get right will be lighting the subject properly since I assume you are already good a composition. Next skill will be shooting since even in the S90 there will be some digital delay. I notice in the S90 manual that Canon repeatedly mentions shooting from the half cocked position, that is an essential technique. Pratice it and do what I always tell newbies, shoot, shoot, shoot! The advantage of digital over film is that its hard to run out of image space 