Having not actually ridden in person, I think the appeal for me comes from the combination of the IP, the ride system, the unique dry-for-wet approach, and out of my general love for charming little C-Ticket Dark Rides and undersea environments.
It's better in person, if a little awkward to get in and out of the ride vehicles if you're a taller person and sit on those little benches.
The ride has great replay value. The scenery is detailed enough that if you're sitting in front of one porthole you don't see everything, so you have to come back and sit somewhere else. The little joystick-controlled flashlight also lets you look for more hidden details.
At 5 minutes or so, it doesn't feel too short. It's about the same as the old WDW ride indoor portion and hits a number of the same beats (squid, lost city, shipwrecks, bioluminescent deep waters etc).
If you ride by yourself it's actually somewhat creepy. It's a very literal dark ride and there's something inherently unnerving about being in a claustrophobic little pod at the bottom of the ocean, but that's part of the adventure. The dry for wet is very convincing too.
All narration is in Japanese, but it's not hard to understand what's going on, especially if you're familiar with the two former American Submarine Voyages.
I love it. I'd put it #1 or #2 at DisneySea, behind maybe Sindbad. I'm biased, but it makes me so happy every time I ride it. My last two trips I never saw very long lines for it compared to other DisneySea rides. Maybe 45 minutes at most. Usually quieter towards the beginning or end of the day like most rides. Obviously, it depends upon when you go.