I also loved 20K Leagues as a kid. Recently, though, while changing some of my Dad's old home movies over to video, I got a look at some of the footage from the ride. While kids would probably still like it, I think adults would find it a snoozer.
I have a feeling that our memories of it might be sprinkled with some pixie dust to make it seem better than it was. The creatures in the water just sat there while you slowly rode past them. Today's technology is far superior. What would be great is an updating of the ride, with either underwater animatronics or even Mission: Space type things added to it.
I could be wrong; IASW is pretty low tech and still as charming as it ever was.