I have the SX20IS, assuming it has similar features and performance, id say for close ups of coral or fish, and go to the mode 'P' (Program). Hit the top center of the scroll wheel were it says 'MF' (Manual Focus) and set it so whatever coral or fish your photographing is in a steady focus. If it won't focus then you need to zoom out.
For WB (white balance), it depends on your lighting. There is a setting for Flourescent and Fluorescent H, which would work well with T5s but for MH i would go with a custom WB.
What kind of coloring are you going for? Cool white, soft white, dark blue, actinic blue?