I'd have to agree with the other guys, you have to get closer, a lot closer, with a focal range of 40mm its tempting to just zoom in rather than get as close as possible (without damaging the environment of course). The 17-40L is a pretty awesome lens, and you should be able to get some real crackers with it, just have a bit of a google around, a lot of great pictures were taken underwater with this lens.
I would recommend that you wait a little before you go out and buy new lens, extensions and possibly a new dome, if your buoyancy is so that you cant get close enough with the 17-40, then a new lens (fisheye) is only gonna make the subject smaller in your picture, a new lens is not going to help on your buoyancy control.
Make sure you have excellent buoyancy skills and then see what you think if your 17-40, if you are feeling that you can't get close enough because of the minimum close focus of the 17-40, then you can change your setup, but if its because of bouyancy then it doesn't make much sense.
It might be a good idea to go with your local dive club to the pool, bring your camera, and practice there once a week (you don't have to do a course to learn it but if you have a good instructor who knows about photography then it can sometimes help) after a few sessions you should be getting the hang of it!
Anyways, thats my 2 pennies worth, and as Tim said, it could be a whole lot worse!