My guess is that the signs say "aquarium" instead of Observation level because ever since the Nemo ride opened, they struggled to get people to actually explore the aquarium instead of just heading straight out. But more likely, the people who made the sign simply just don't get it.
It bothers me how much potential the pavilion has, even with the Nemo tie-in. Especially with the aquarium being a huge setpiece in Finding Dory, there's really no excuse. The ride itself could have been an educational adventure, like a field trip with Mr. Ray, or anything other than what they did, which was go for the easiest, laziest possible concept of covering the cliffnotes of the movie.