I have a Panasonic Z53, and use the Panasonic DMW-MCTZ7 marine case when I take it to go scuba diving. I'm very happy with both Z53 (although, of course, I'd buy a later model if I were to get it now) and the marine case. I haven't had any leakage issues, although I have had trouble with temporary fogging twice, even using the supplied silica packs. If you plan on diving, instead of just snorkeling, you will need to get a marine case, even if you go with a camera that is waterproof on the surface.
One thing to think of, though: every time you replace your cameraeven if it's just upgrading from one Panasonic model to its successoryou'll likely have to buy a new marine case too, since the measurements of the cameras do seem to change. I've had some modest luck selling old housings on eBay, but certainly not enough to make up the difference. Now every time I buy a new camera, I essentially calculate that I'm buying two, since the housings aren't exactly cheap, even if they're just plastic.
