You never really “interact with the characters” in a book report ride, nor do you travel especially more “around” anything in Under the Sea than in any similar attraction. The attempts at interactivity are often what feel forced (see TBA).
IMO, the issue is staging, the static nature of a bunch of the supplementary figures, and the massive narrative leap at the end. The first two Ariel figures are posed and positioned supremely awkwardly, which gives a really poor first impression, and the static nature of the fish really takes you out of the experience because in order for anything underwater to believable, it has to be moving pretty much all the time. This is a case where I think much more extensive projection mapping could help a ton (even at the expense of some of the current static figures), plus a rethink on placement of a couple of the central animatronics. The problem of flow at the end is largely unfixable, but I actually think the attempted improvement of showing Ursula dying does more harm than good. I might just put the focus more on the loss of Ariel’s voice and its return. Change the screen with the bad graphic of Ariel getting legs right after Ursula to instead show her voice being absorbed by the nautilus necklace, then make the scene after Kiss the Girl into a set of proper Eric and Ariel animatronics standing amongst fog on the ship deck, her voice returning to her from the necklace.