I have been recording digital images and sounds for over a decade now, with cameras and DAR's (digital audio recorders) using a pile of different memory sticks. None has ever failed. The only problem I ever had was a card formatted in one device that wasn't readable in another device; reformatting by computer fixed that. Cards ten years and older still work fine.
I repeat, none has EVER failed -- which doesn't mean that they can't, just that they haven't (yet). I don't buy memory cards on eBay, but name-brand cards at WalMart or CostCo or major online retailers. And before I stick it in a device, I run diagnostic tests on each card via computer. That works for me.
I have one underwater camera, an Olympus 770-SW that can survive down to 10m, according to specs. It has nice tight seals protecting its 8gb xD card. I would be VERY nervous about using smaller cards and swapping them during a wet shoot. I only pull that card when the camera is nice and dry and sitting safely next to a computer on a desk. On all my devices, I use the largest possible card, so I don't have to bother with field-swapping. But just in case, there are extras in my bag...