It is beach & then hard pan - a few but not many urchins. Boots recommended though at a pinch you might just about get away with bare feet. Preston Bay is ironshore - not too jagged but boots definitely needed. If you are day shore diving Cumber's Caves/Bus Stop is a good start as it is easy to navigate - the buoy is on the miniwall (not a long swim) - head across the sandflat to the wall, down one of the chutes, tunnels out onto the wall, turn R or L along the wall for a wall then return via the sand flat to the miniwall - even if you dont find the buoy again if you then turn shorewards you will reach the beach over the hard pan doing your safety stop as you swim in - also somtimes nurse sharks in the shallows.
The thing I don't like about the boat night dives was that they were after dinner so that after dinner & the boat ride there & back it made it very late (& I never eat before a dive much). What we do is get of the plane, walk to Perry's, get the car, go to the condo, put the beers in the fridge, get the tanks, straight to the dive, back to the condo, 5 mins in the jacuzzi & then down the Iguana in time for last orders (for food) at 9. Then on into the Little Cayman nightlife ...