[JBen]

Hazratty.
Do you shoot in Raw? Doing so allows you to alter/adjust the white balance in post for accurate colour reproduction.

Using scene modes is a lot like shooting in A (aperture priority) or T (Shutter Speed priority) depending on which is the critical aspect. My body only has manual functions so I can only guess at what settings might be chosen when in scene modes (no way to choose and see). Typically technical aspects remain the same though.

Sport
Your telling the camera you want to shoot action and want stop action settings. So the camera sets a factory default high Shutter speed. You can do this in TV by setting the SS to 1/500th, 1/1000th or 1/2500th.

Landscape:
Your telling the camera its a landscape and the camera will by default choose a small f/stop for Depth of field. Little different than setting your f/stop anywhere between f11 and f16 in AV

Portrait.
You want only your subject to be in critical focus. So the camera will open the lens as wide as it can. You can do this in AV by setting the lens to f4.0 or f2.8 or f1.4 etc

Underwater.
My Canon S95 does have this scene mode and from what I recall the priority in this mode is White Balance where it tries to accommodate the light spectrums.

The technical aspects and principles of photography remain the same no matter the genre. By trade Im a professional (and freelance) photographer in the outdoor lifestyle markets. I shoot for a few magazines as well as freelance. Whether Im shooting people camping, fishing, hunting, whatever I can apply the same principles to Weddings, Sports (hockey etc) or underwater aquaria. The "artistic" side is what sets one wedding or landscape etc photographer apart from another and that largely comes with time, practise, etc. I know theres a few here who already have this down pat and hopefully they'll see and chime in on this for you.

If you have no means of altering White Balance in post and have to shoot jpeg, Id stick with underwater mode for truer colour reproduction, landscape mode for FTS and portrait (possibly) mode when shooting individual fish/corals etc. If you can shoot in RAW and have the editing software, do so and try using AV/TV to start. Really the only difference (well not really, theres different ways to meter etc) between the scene modes and the semi auto modes is that in Av/TV "you" can fine tune the shot based on what your seeing, and then take another where-as in a scene mode you can't.

Cheers,
J