The comments about 20k going underwater and NOT going underwater are BOTH right. The submarine was not a free-floating vehicle. It was on a track, and it was partially submerged so the riders had portholes that were a foot or two beneath the waterline (the "captain" on top had a view above the water).
It certainly wasn't fancy by any means. You would basically be going around and then come across a giant squid (that would wrap the tentacles around the ship), see some mermaids, ultimately you would discover the lost continent of Atlantis. Naturally it was hokey in the sense that no matter how far down you were sent (and the air bubbles past the porthole were cool) you always saw the waterline above you.
However, it was a unique attraction. It was very popular too as the lines were always significant (granted, the capacity wasn't all that great). I think it was a D-ticket under the old ticket system. But it's sorely missed as it really was a memorable ride that you just couldn't find anywhere else.