Guardian farm to produce prismarine and prismarine crystals, although it also produces ink sacs and nautilus.

To make it work you have to build it on top of an ocean monument, in the same way as the classic farm, with the outer wall of the circle aligned with the water level. Obviously you will have to drain the sea and remove the monument with the farm.

If you want to see more decorated farms like this, go to my profile and follow me on my YouTube channel.