Don't know the exact number, but, on the Magic class ships, the Quiet Cove pool was at least 6 feet deep at the deep end. My head was underwater when standing on the bottom there.
Not sure about the Donald/Mickey/Goofy pools.
EDIT: Just checking some of my pictures. The Donald pool on the Fantasy is 5'3" Max depth. The Mickey pool (Wonder) is 1'6". Quiet Cove (Magic class) 4' at shallow end, deeper at other end.
I'm 5'9" and I was shocked how deep the Donald Pool was. I wasn't paying attention to the actual depth and jumped in and my 3 year old didn't wait for me to turn around before practically pouncing on me so I struggled to keep myself up and push her to the side to hold on. Here is a link to an article that was here on the DIS: http://www.wdwinfo.com/disney-cruise-line/recreation-pools.htm
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