Actually, composition is something you
always have control over. You may not be able to control the
environment, but you can always use good composition techniques to minimize the distracting elements. You may not be able to get rid of them entirely, but depending on what your intentions for the photos are, that may be okay.
In terms of exposure, these feel really really dark to me, by at least a couple of stops. Lighting is something that you definitely don't have control over in an environment like this, and that means you have to compensate for it by opening up the aperture, increasing the ISO, or using a longer shutter speed. Since you're not carrying around a tripod, changing the shutter speed is not really an option (you'd end up with motion blur), and from what I can tell you were probably at the open end of your aperture, so that only leaves ISO as an option.
In terms of focus, they're a bit soft. I don't think that's a focusing issue so much as a shutter speed issue, combined with the camera being hand-held.
If you're really interested in getting in to photography, I recommend you get some practice away from this kind of environment. Do some reading on the exposure triangle (shutter speed, aperture, ISO), and take a bunch of crappy practice shots to test out what you learn and understand it better. Then, when you end up at the aquarium again, you'll have some additional knowledge to help you make better choices about your camera settings and such.
