What water rides are you talking about? I took my DSLR on Splash Mountain, and just held it up in the air during the big drop... I took it on Kale River Rapids and they have the middle of the raft that has a plastic cover... It is not the greatest protection, but it can work...
To better protect my gear, I would just bring a 2 1/2 gallon ziploc bag to put it in during those rides... That seals and protects it completely from splash zones... I wouldn't trust it enough to submerge the camera under water, but a splash shouldn't hurt anything...
Based on someone's advice here on the Photography board I bought an Optech Rain Sleeve from Adorama for a few dollars.
It worked quite well, it doesn't seal tightly at the lens but if you have it reasonably well installed around the lens you could pour water over the thing and not get water into the camera - depends on how water tight your lens is as to how much water might get into it.
But even with a pretty hefty splash on Splash Mountain (why do they feel the need to shoot a water cannon just to catch MY boat when you go around the 'bend' after the big drop?) there were no problems with water getting to my camera.
I purchased from Cabelas the Sea to Summit Dry Sack (8 litres). It easily holds my Pentax K10d w/battery grip and 28-70 f2.8 lens. They come in many sizes. They are waterproof, but I would not want to submerge them for any great length of time. For WDW's wet rides they work great.
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