Ah, yes, Nemo, home of the "Nemo Stampede!"
Since it reopened as Nemo's subs a couple of years ago, it's been very popular. However, it's a very slow loader, so the lines are almost always the worst in the park. Therefore, it's engendered the stampede that happens nearly every morning as people try to hit it first before the lines grow huge. If you get to the gate 45 minutes before opening, then walk briskly straight for Nemo as soon as you get through the gate (or through rope drop at the top of Main Street) you might make it on within 1/2 hour. A lot of people just plain RUN, despite CMs all along the route calling, "No running, please!" People have reported being trampled by strollers, or alternatively, that people actually vaulted over their kids in strollers running for the subs.
We used up one precious Magic Morning to do the Nemo Stampede. We made it onto the first sub. But it wasn't worth it, IMHO. It's a cute ride, but we were up and at the security stations by 6:10 a.m. (for a MM start of 7 a.m.).
My kids liked it, but weren't interested in doing it again. DD13 said she had "personal space issues" since you're packed in like sardines on very narrow seats. If you're claustrophobic it would be a bad scene. As I said, cute, but not really worth all the fuss.
There is an alternative: they have a viewing room where those with disabilities, who can't get onto the subs, can view the same video as runs in the subs. You can ask the CM and they will direct you there; for families with children who can't wait in line that long, it's usually much less of a wait. Some people wait in line anyway to do this; but others have reported that you can just go directly to the viewing room without waiting. Anyone can use this option, not just those with accessibility issues.
Hope that helps!