You can purchase tickets at the door but we highly suggest you order them online and skip the long lines that often form. In addition, they get really busy and we recommend that you visit very early in the morning or after 4 PM in the afternoon and try to visit them during the weekdays if you can.
We decided to visit this time during a weekday afternoon and it was much less crowded than it was the first time that we visited. It was a LOT nicer this time because we were able to obtain easy parking and we could really take our time walking from exhibit to exhibit. We didn't feel pressure to hurry along and because they weren't crowded...we had better views of the animals.
They have a brand new exhibit called "Living Seashore" which is a hands-on exhibit where you can touch jellyfish, snails and other animals. This is the ONLY area where you are permitted to touch the animals...so please remember that as you are visiting. In addition, you are NOT allowed to tap or beat on their aquariums or walls to get the animal's attention. Please be respectful.
After you are done visiting the National Aquarium in Baltimore there are plenty of other things for you to see and do within walking distance from the Aquarium. You can rent paddle boats and take them out for a spin, take an evening dinner cruise by boat, visit the Ripley's Believe it or Not Museum, take in an Orioles baseball game and more! There is something always going on nearby!
You can learn more about the National Aquarium by visiting their site online. You can get their hours of operation, schedule of events and even reserve/buy your admission tickets online.