What's the difference between 蝦 and 鰕?
You should use 蝦. As can be seen in zdic.net 鰕 used to mean shrimp, but is not used anymore in this meaning. In Japan it however is still a common character.

Using Google:


蝦 site:tw has about 7,060,000 results
鰕 site:tw has about 29,400 results


The latter is used in some exotic fish names.