I love your tank, it is very beautiful. Your camera isn't a Canon "Powershot" at all, it is actually a Sony Cybershot DSC-S500.
DPReview doesn't even have it listed as a Camera

, this is the first time I have encountered this.
First lets look at the Specs:
-6 megapixles (this is fine, anything over 7 I consider wasted overkill unless your are heavily cropping or blowing the images up large)
-1/2.5 in sensor (Ouch, this will hurt your realistically attainable image quality)
-ISO 80, 100, 200, 400, auto (this is the levels of sensitivity that your sensor can capture light...high numbers deliver faster shutter speeds but grainy images. Your small sensor size will make extra grain compared to a large frame DSLR or top of the line point and shoot camera)
-3 x Optical zoom (good stuff, use it freely and creatively)
-2 x Digital zoom (ick, keep that digital zoom down...using it will sacrifice a lot of quality.)
-Exposure: Automatic (my heart sunk when I saw this one...I don't know how much I'll be able to help you with automatic exposure. Basically your camera is going to do everything for you, even if you don't want it to.)
-White balance: Automatic (my heart sunk even lower, this is why your tank looks blue)
-Available Image Formats: JPEG (I didn't expect RAW, but was hoping for at least TIFF...throw us a bone!!)
Sadly I don't know how much help anyone is going to be. You have a fully automatic camera with a mind of its own, and it doesn't have much elbow room to work with. As much as I hate to throw in the towel, your going to need to look for a new camera if you want better pictures.
That said, lets look at what you got...
1)
Exposure:
-Aperture = f/2.8 (your camera did it's best here...I'm pleasantly surprised it can even go to f/2.8. This low F/# is why the front and back of the picture are blurry. Being a still shot of a coral, f/5.6ish would have been my choice with a tripod but you can't tell your camera what to do. Like a little kid "your not the boss of ME".)
-ISO = 400 (this is why you see the black specks...I would have used ISO 100 on a tripod)
-Shutter Speed= 1/30 of a second. Your camera chose f/2.8 and ISO 400 trying to get this number lower (faster shutter speed)
Cameras see light differently. It thought it was doing a good job, but again, the "your not the boss of ME" syndrome comes into play and you just can't tell it otherwise. Everything is fully automatic. This is why your picture is blue.
2)
-Aperture = f/3.9 (it should have used f/2.8 on this one)
-ISO = 400 (hey, it got that one right)
- Shutter Speed = 1/125 (it hit 1/125 and said "lets try to get more of the frame in focus", so it bumped your aperture down a notch...I would have turned down ISO instead)
3) I like this shot, it looks pretty good...but thats more of your tank's great character and less due to the image quality.
f/2.8
ISO 400
1/160
You got a fast shutter speed...but for a still subject where it isn't necessary. You could have turned that ISO down to 100 to recover some image quality and hit f/4 or f/5.6...oh wait no you couldn't have, its a fully automated process.
Basically you have a great tank but a crappy camera. You didn't do anything wrong because...well...you can't do anything. It looked at the situation and did the best it knew how, but you can't make it do what you want. If you want to take better pictures, your going to need a better camera. I wish I had better news.
-Mike