I have this phone. Stick to the main brands--not the unknown Chinese cheap crap.

OTTERBOX https://www.amazon.com/OtterBox-COMMUTER-Samsung-Galaxy-Packaging/dp/B083YS9S53/ref=sr_1_4?crid=1Z76GDVXDFF8B&dchild=1&keywords=galaxy+a51+4g+case&qid=1625721874&sprefix=galaxy+a51+4g%2Caps%2C164&sr=8-4

​

UAG https://www.amazon.com/URBAN-ARMOR-GEAR-Feather-Light-Protective/dp/B085SWPP4M/ref=sr_1_1_sspa?crid=2SSQXHP3YO2QW&dchild=1&keywords=samsung+galaxy+a51+4g+uag+case&qid=1625722154&sprefix=galaxy+a51+4g+case+UAG%2Caps%2C151&sr=8-1-spons&psc=1&smid=A3DVC6WBVNJ5RS&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFSN0NFMUgxSkxITEImZW5jcnlwdGVkSWQ9QTA2ODExNjgxUjFSQlhWQjJMTzFJJmVuY3J5cHRlZEFkSWQ9QTA5NjE3NjIyV0c4QVVMVkdWM0RDJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

SPIGEN https://www.amazon.com/Spigen-Tough-Designed-Samsung-Galaxy/dp/B0832PG669/ref=sr_1_3?crid=36UE6NN61JG6W&dchild=1&keywords=samsung+galaxy+a51+4g+case+spigen&qid=1625722213&sprefix=samsung+galaxy+a51+4g+SPIGEN%2Caps%2C149&sr=8-3

RINGKE https://www.amazon.com/Ringke-Fusion-Designed-Galaxy-Case/dp/B0834K7B29

None are waterproof, but all are very protective. Especially the UAG, the SPIGEN, and the OTTERBOX.