Sea Life Adventure is a small, independently-run aquarium on the seafront immediately west of Adventure Island. It has about 30 displays divided between local Thames Estuary species (sea bass, stingray, crabs, native sharks like the smoothhound), tropical reef tanks, and a small Amazonian section with piranhas and tetras. The walk-through reef tunnel is the headline display.
It is small enough to do thoroughly in 90 minutes. Open daily; combined tickets are sometimes available with Adventure Island next door.