Hi Beanstir,
Based on my experience both underwater and on land, the speed of either SD or SDHC card really won't affect your ability to take videos. You'll really only notice the speed of a card when either 1) using continuous photo mode where you're taking 1-3 photos per second and 2) when transferring your files to your computer.
I have had normal SD/SDHC cards, as well as high speed SD cards, and never ran into any limitations or frame drops when recording video at all. To be honest, I don't even really notice the difference when transferring files. For me, I only notice when in "burst" mode where the regular speed cards slow down after a few seconds. I can't imagine you'll really need this underwater - if you see something really cool with action, you'll likely just take video!
So I would suggest for the same price, get a higher capacity card over a theoretically faster card...
Cheers!
Bryan