Underwater Lens for EOS 600D
I do not know what kind of waters you plan to take pictures in (clear tropical or colder dirtier), but I always recommend to start with macro. A 50-60mm macro lens and an Ikelite strobe with TTL should serve you well. After that a full-frame fisheye be a good challenge. Just remember you need to get extremely close with underwater photography for sharp pictures. Hope this helps.
Been using Ikelite products since 1970's and great value and service always.
Take only pictures. Leave only footprints.