Why isn't a wet sense of humor the exact opposite of a dry one?

(Why)

Many years ago Mrs Didge and i were swimming at Bondi Beach when we were picked up by a whopping great wave and dumped. I got back to my feet and patted her on the bottom then realised I was patting bare skin. I looked down and saw that her bikini was around her knees. I looked up and saw that it was another woman. 

I looked around in panic, found Mrs D behind us cackling (she had seen what happened) and stammered, "Th-that's my lady back there." Fortunately the woman was old enough to laugh about it and I escaped with a story to tell. 

Now that's what i call a wet sense of humour. 

For the same reason a dry dream isn't. 

Good question...anyone have a good "wet" joke??