How many shots on a card is more going to be determined by your battery life than the card, with any decent-sized card... at least if we're only talking about one dive. Just swap in another, empty card while on your surface interval, while you also swap in fresh batteries (even with rechargables, it's best to take fresh batteries down). If you don't have two cards (at least), hopefully you can dump them to a laptop or something, else you'll be treating memory cards like rolls of film with no backup. It's an option, but I wouldn't feel safe. I usually dump to the laptop every 2 dives or so, and burn off a CD or DVD in the evening to be better protected yet.