Bring 4 liters of water to a boil. Reduce heat to medium-high.
Add frozen shell-on shrimp. They will sink to the bottom of the pan and the boil will stall.
The shrimp will float when they are done cooking, which is also typically around when the water returns to a boil.
Remove shrimp to a strainer over a bowl to cool before peeling. Do not rinse.