Short answer: It is the UK's largest aquarium and every ticket includes free re-entry for a year, which changes the value completely if you live within reach. It is closed on Wednesdays — the most common way a visit goes wrong. Allow two to three hours for a first visit.
This is the thing most people do not realise until they are at the desk: admission includes free re-entry for a year. If you live anywhere within an hour of Plymouth, you are not buying a day out, you are buying a pass at day-ticket money. It also takes the pressure off — you do not have to march a tired four-year-old round the last three tanks to get your money's worth.
Worth repeating because it is the single most common way a trip here falls over. Check the day before you drive in, particularly in school holidays when people assume everything is open.
It is the largest aquarium in the UK, and the scale of the main tanks is the difference between this and a provincial attraction. It is also a working marine conservation organisation rather than a themed attraction, which shows in how the exhibits are explained.
It sits across the water from the Barbican, so it combines with a Barbican morning without moving the car. Barbican parking is expensive — Devil's Point gives three hours free a short way west, though that is a longer walk in.
The Box is free and closed Mondays, so an aquarium Tuesday and a Box Thursday covers two days of weather-proof city without clashing with either closure. The Mayflower Museum is small, about an hour, and aimed at younger children if you want a third.
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