You should only make ceviche with the absolute freshest shrimp (or any fish or seafood). The success and excellence of ceviche start with the quality of the fish. If you are lucky enough to get fresh shrimp right from the water, hooray! This is a great way to make use of them. However, know that most shrimp sold even at seafood counters was previously frozen and simply defrosted before being displayed on the ice. Make sure to defrost it safely and use the shrimp right away.