I remember seeing this movie when I was a kid. And thinking it was awesome. When you're eight, that's probably accurate. Now, less so.
There are parts I still quite like. The underwater house is a sweet slice of mid-century retro-futurism. A late sixties vision of our aquatic future with faux-wood paneling and a push button lifestyle. And the musical numbers (which there are a lot of) aren't horrible for 1969. It could just be the repetition, but they do worm their way into your head.
Otherwise, the movie is pure fluff. It's not laugh-out-loud funny, but it's a harmless distraction. It's got about six plots going throughout, all crashing together in the last ten minutes. Eventually, the movie gives up and abruptly rolls its credits.
Still, if you want to see Richard Dreyfuss menaced by large sharks, this is probably the place to go...