The UK's biggest – and perhaps best – aquarium is lodged beside the Barbican harbour. There's a huge amount to see, but the highlight is the impressive Atlantic Ocean tank, the deepest in the UK, with over 2 million litres of water and a population of sand tiger sharks, lemon sharks, barracuda and rays, as well as a replica of a WWII Walrus Seaplane. Other zones explore the Great Barrier Reef, and local habitats including Eddystone Reef and Plymouth Sound.
Tickets are 10% cheaper if you buy in advance online, and remain valid for a year's entry.