Well you're not shooting whales so you don't need an super wide lens as much. Even with whales, fisheyes give them that guppy look.
I think the Sigma 12-24 is very useful for video... not great for stills because of the lower resolution but 12mm is awesome.
The only caveat of shooting with Canon DSLR is that darn 4GB file limit. It forces you to be vigilant about recording time and also forces you to watch for activity more intensely.
12min limits is no problem...I can hold my breath and swim for 60 seconds at the most!