A waterproof two way radio is usually one that meets mil-spec standards. Buying a product that meets mil-spec standards means you are buying a quality product that is built to withstand tougher use.
In an effort to achieve standardization objectives for purchases of products used by the military 
and other government organizations, the U.S. Department of Defense created standards that these products must meet. You will see these standards called military standard, “MIL-STD”, or “MIL-SPEC.” These standards ensure products meet certain requirements, commonality, reliability, compatibility, and similar defense-related objectives.
The Department of Defense also has standards for two-way radios and radios that meet this standard must meet levels for such criteria as vibration, rain (water), salt air, sand/dust, shock (dropping), humidity, temperature as well as other factors.
You may also see radios with an “IP” designation. IP stands for Ingress Protection and it is an international standard that specifies the sealing effectiveness of the enclosure of a product. It is a two digit code that tell you how effective the enclosure is to penetration from fingers, dust, and water.
For instance, for a radio that meets IP 55, the first 5 means it is protected from the amount of dust that would interfere with its operation. The second 5 means that it is protected from water that shoots from a nozzle.
So when you look for a waterproof two way radio you really want to look for one that meets mil-spec or IP 55 standards.