I lived and worked( at Sunset Divers) in Grand Cayman for two years (1999-2001). Some of the dive sites that are favorite
Devil’s Gotto - shallow dive, but well worth it
Aquarium- lots of fish
Trinty Caves
Orange Canyon - lots of orange sponges
Few dive operations can reach the north side, but if you get there, Babylon shouldnot be missed. Lots of black coral and strawberry baskets.
Sunsets Divers offer the Manta dives - it’s a 3-dive all day dive on, the North Side, weather permitting.
Hope this helps
Shirley