I'm not sure what your Canon uses, but the new DC800 can take an SD card up to 4GB and has a 1250mAh battery so you should be able to get
plenty of video.
Thanks! Maybe I didn't mention it here, we discussed it some on a thread on your Manufacture forum, but my Canon A540 and my Sealife DC500 both have a 2 Gb card limit. I bought a 4 Gb card not realizing that cameras had limits and neither will recognize it.
Then I learned about HC: High Capacity cards. Walmart.com advertises several of them without any warning but many dissatisfied customer comments.
It would be nice if manuals stated the card limits; neither seem to. Your manual for the DC500 just suggests a 512 Mb card.
We have some videos taken with the DC500, DC600, and DC800 up in our video gallery for anyone interested in taking a look:
SeaLife Underwater Cameras