Underwater Lens for EOS 600D
This is exactly correct. Get a macro and a fisheye and the ports to match. If you are unfamiliar with the term port, it is the "wet" portion of the lens and corrects for the different refraction rate in the water and different types of ports are required for different lenses. Can get pretty confusing, but ikelite will set you straight.
I would also say a strobe is THE most important thing you can add if you are diving more than 20ft deep in crystal clear water. If your choice is either a macro lens and compatible port or the stock lens and port plus strobe, stay stock. The strobe is critical. Just get one for now though, add a second strobe later.