With memories cards nowadays, you can find cards as big as 8GB and 10GB. That size is enough to last you an entire trip if you don’t always shoot on the “fine” setting. However, most professional underwater photography experts will say it is not a good idea to have all your pictures on one card, especially with underwater photography. Using a different card every couple of days or even every day ensures that you will have pictures if you unfortunately flood your camera on a dive. Sometimes with a flooded camera the pictures on the card can be salvaged but only if it is a minor flood. Remember, flooding the camera isn’t a matter of “if” but a matter of “when”. Protect your memories the best you can.
Side Note: Make sure to always format your new card before you use it. Some cameras will not save pictures on a card that is not formatted.