The only value to the faster cards IMHO is in downloading shots to the computer. The buffers on these cameras (even the D70) are much deeper than your strobes can keep up with. I doubt that even in fast sequence shots like the diver entry sequence that Alex showed the buffer did not fill. So the card speed never played into the equation. In rare instances when you want a huge sequence you can always shoot JPG. This is particularly true of the D200 20 shot buffer, but even the D70 4 shot is enough.
So if you don't mind waiting for your computer you can go with the cheapie sandisk basic model. Pricegrabber shows the 2GB Ultra II cards at $169 and the standard lexar at $133, so for $30 more you can download faster. A good deal if you aren't buying more than 2 cards.
Get the size that allows you to shoot a full day's diving with your camera and your shooting style. For me 2 GB should be good for 4-dives with D70 RAW+JPG. I wouldn't worry about loosing your shots. It hasn't happenned to me so far. My bigger fear is ruining the camera by opening the housing on the boat, with drippy wetsuits and a rushed/dirty environment to put it all back together. I think your chance of a flood due to user error is more likely than loosing images.
I use 1GB cards because I only dive 2/day. The afternoon is reserved for playing with the kids on the beach. Everything in moderation.