Yes, adults do have fun on it. I know all the words and like to sing along and try to steer my fish so that I get wet. It makes me feel like a kid again.
I have only gone to the parks once with children. Most of my visits have been with another adult. We love one fish two fish. We are usually the only fish with two adults in it, but we don't care!
I'm glad I'm not the only one who just loves this ride. Yes, I know all the words too! We went in early June before the hotel express pass had changed and I know we must have rode that ride a dozen times. My kids are at the age they can ride all the big rides now and they tend to turn their noses up at what they call "baby rides". We had made a deal before vacation since this was our first trip to Universal and IOA that we would try to ride everything at least once. They actually had fun on this too! But I wouldn't dare let their friends know!