I'm certainly no expert or anything, and I don't have a dSLR myself either. However, I do have some thoughts on your self-portrait. I don't understand why you say "...and I know the background's busy, so please don't comment on that." If you know the background is busy then why don't you do something about it? As someone else mentioned, fill the frame with your subject (in this case, you). Crop out as much of the background as you can. And while you're at it, crop out some of your t-shirt as well. It's just a big black spot and adds nothing to the photo. I've saved photos in the past that I thought were too busy by cropping. Give it a shot. That's another one of the wonders of digital photography, you can try something in Photoshop, and if you don't like the result just undo all your changes.
As for number 4, I'd take it a step further than what others have suggested. To me even the other swimmers are distracting. I'm at work, so I can't try it myself, but I wonder how this one would look with nobody but the one swimmer in it. And I think it would make it more interesting if you don't have every single drop in the frame. Again, try it and see what you think. Trial and error is one way to learn new things. If you don't like the result, just go back.
On number 3, I like how you captured the moment. It really gives you a feel for the scene, and I like the composition. Even in this one though, the girl in the background with the pink shirt is a bit distracting. Is this someone who you know that you might have future study sessions with?