Minecraft wiki entry for Turtle_farming, associated text: A barrier (the block floating in the air) to prevent mobs from entering the farm from the sea while allowing turtles to pass underneath
A solid seawall to keep the ocean or body of water out
A top slab that allows large turtles to bridge the gap between the barrier and the water block, allowing them to reach the beach when entering, or reach the seawall when leaving
Hoppers to collect the scutes dropped by baby turtles as they grow
Water source blocks arranged to create a waterfall toward the hoppers. Turtles cannot easily swim against current, so it is important to have a span of at least two water source blocks on each layer to provide a region of still water in which the turtles can move and climb out onto the beach.
A sand beach to allow the turtles to spawn