The large, state-of-the-art Atlantis Marine World aquarium (431 E. Main St., 631/208-9200, www.atlantismarineworld.com , 10 a.m.–5 p.m. daily, adults $15.50, seniors and children 3–11 $13.50) houses everything from native Long Island  fishes to moray eels, Pacific octopuses, piranhas, stingrays, and seals.
Near the entrance reigns a figure of Poseidon, king of the sea, while further on is a large live coral reef, awhirl with colorful tropical fish. There’s also an underwater cavern that’s home to more than a dozen sharks, and about 80 other exhibits. Visitors can embark on a simulated submarine dive and take in a seal show.
The aquarium was built around the Riverhead Foundation for Marine Research (467 E. Main St., 631/369-9840, www.riverheadfoundation.org ), which maintains several exhibits on site. Among them are a center for rehabilitating injured seals, sea turtles, and other marinelife and a touch-tank where children can handle starfish, crabs, snails, and small fish. The Riverhead Foundation also offers seal-watching cruises that depart from Point Lookout on the South Shore  and naturalist cruises down the Peconic River.