In 2013, the aquarium completely overhauled its centerpiece, the aptly named Giant Ocean Tank. The 200,000-gallon tank, encircled by a four-story spiral ramp, is now home to an enlarged Caribbean coral reef and more than 1,000 fishes. The best news for visitors is the most apparent: During the overhaul, dozens of viewing windows were replaced with larger, clearer panels. Together, tanks throughout the building house more than 30,000 fish and aquatic mammals, starting just before the entrance with a colony of Atlantic harbor seals, identified on panels above their enclosure.

Once inside, the first large exhibit is home to rockhopper, little blue, and African penguins. Turn left to visit the 25,000-gallon Shark and Ray Touch Tank, where the expansive water surface makes some of the resident animals accessible to visitors who want to “pet” them. Also on this level is a new exhibit, the Blue Planet Action Center. It highlights the numerous threats modern human society poses to the oceans, but it’s not all bad news—visitors learn about possible remedies and view shark eggs and babies as well as (in a separate enclosure) baby lobsters. Climb the ramp to the third level to reach another touch tank, the Edge of the Sea exhibit, with horseshoe crabs, sea stars, and sea urchins. Or head for the back of the ground level and the open-air New Balance Foundation Marine Mammal Center. The resident colony of fur seals and sea lions—again, look for their names on the identifying panels around the exhibit—is especially interesting when trainers are working with the animals. Look for the schedule (usually three sessions a day) when you arrive.

In an adjacent building is the separate-admission Simons IMAX Theatre ★★★, which screens 3-D films. Check ahead for the schedule, and consider buying a discounted combination ticket.

The aquarium is worth at least a half-day visit; try to get there early. On weekends year-round and every day in the summer, lines can be long and crowds uncomfortably large, leading to frustration and, often, tears. Consider investing in a Boston CityPass or Go Boston Card.