I have one of the pentax OptioWP cameras and have given it a hell of a lot of abuse in caves. It's actually only waterproof to 1.5m not 5m and only for 30mins at a time, but like you say, it's enough to keep it safe splashing around. It also means you can wash it off when it gets covered in mud!
Olympus has just brought out a similar thing... the 720sw... i think. It boasts over 7M pixels, and is waterproof to 3m for continuous use. It also says it's shock proof for drops up to 1.5m, although i'd like to see the results done by any caver! Apparently it also has a special mode for taking photos in the dark.
I'd like to see some results...
As far as only using the screen for shooting, I've had no problems with the pentax (the olympus also has no viewfinder), in fact it generally helps with steam problems, being able to hold the camera at arms length.
The only real quibble i've got with the pentax is it's double flash, which means the simple slave units won't work with it, and the flash is very close to the lens so you do often get a nice photo of steam!
Oh, it also fits really nicely in one of the really small peli cases, so that protects as well as you can until you need to use it, and you don't mind taking it through that tight crawl!