I really hope that you can use other apps when underwater. Just spitballing I thought of a number of ideas, and I'm sure that over the next year the really smart people will come up with even more.
One that just occurred to me is communication. Divers sign language is often inconsistent between different areas of the world or between different people with different training. Basic simple stuff like thumbing a dive is fairly universal, but imagine an app with a few hundred phrases that you could preload, customize and scroll through, to display for your buddy.
I don't think that in 2007 most people - even tech savvy people - could have anticipated all of the applications that would someday be used on an iPhone. I hope that this follows in that tradition.
The big barrier to other apps (besides any "risk" factors of moving away from the Dive App) is that the touchscreen is disabled under water. The ability to switch to a different app and use it without the touch screen is limited. Certainly, your use case of scrolling through phrases probably could be done without touch (although picking which app to start might), but at least at this point, there isn't much that is done without touch and app switching (outside of the new action button) requires it.