Tony...don't let the DRebel mislead you. It's a wonderful camera and can even be comparable to the 20D. I have both and while the 20D is an excellent camera, the 300D can't be beaten for it's price, imo. Just didn't want you to be embarrassed when you mention that you are using one. Yes, shooting through glass is extremely difficult especially when the other side is full of water.
Here are some tips that will help you if you are still having issues. If you know these already then just ignore the rest.
1. Use a tripod. You can pick up a decent sturdy tripod at BestBuy or Walmart *shudder* for about $30.
2. If you are shooting coral, set the camera in manual mode and adjust your ISO setting to around 200-400. This will let you get more light in the camera but it will introduce some grain. You can also get as close to the glass as possible while zoomed all the way out. Then set your aperture as low as it goes...ie 3.0 This will allow you to use a faster shutter speed so that the coral doesn't blur. Then just adjust your shutter speed til the light meter on the camera is in the middle. You will probably get some really hot spots on your sand bed but i wouldn't worry much about that.
3. If you are shooting fish, then you want a faster shutter speed so you can do the above and adjust your ISO speed to around 800. You will get grain with the DRebel at 800 ISO.
4. Turn off your pumps. This will keep air bubbles and coral from blurring your photos.
5. Use a tripod :cool1:
6. Adjust the tripod so that you aren't shooting at an angle to your subject. This is one of the harder elements to achieve because our subjects might not always look the best from the front angle. Just experiment and see how much of an angle you can shoot without it distorting the image.
Hope that helps a little and if not then hopefully it will help a newer photographer.
(nothing compared to your camera) lately and it sure is tricky shooting through glass. I seem to be getting better though, with time and practice.