Having been in your situation, I can safely say there are no perfect answers. I had an xTi (400D) and was looking to do underwater photography. I ended up deciding it was not worth it to get an underwater enclosure that cost more than the camera itself, so decided to wait until I could get a nicer camera to be worth it.

In the interim, I went looking for devices that would handle my needs and came down to going between the GoPro and the SeaLife DC1400. While the GoPro is very ruggedized and simple, it also lacks much in the way of controls, anything in the way of zoom and lacks the underwater specialization of the SeaLife cameras. It isn't a bad choice, but it lacks the support for underwater flashes which are absolutely critical to having clear color at depth. (Though you can use constant output lights for things up closer.)

The Sealife camera ended up being the winner for me because it has white balance modes specific to shooting underwater at various depths and includes a solidly built enclosure for a reasonable price. The quality is good enough and it makes a nice stop gap until I can get my underwater enclosure for my 5D Mark iii. It also supports use of flashes and is far easier to control and use underwater as it is designed for the purpose.

With a GoPro, you will basically clip it to your mask and record whatever it happens to get. With the SeaLife camera or something similar, you will actually be able to capture what you want in the way you want with far greater flexibility and control.

There isn't a right or wrong answer here, and certainly the underwater enclosure for your DSLR will get the best results by far, but it is a question of how much you want to spend and how much use you want outside of water. (The Sealife can be used outside it's enclosure, but it is then just an slightly basic P&S.)