This is a good question and discussion to have. Apart from the mechanics of changing a synonym. What would the purpose of "waterproof" be. On one level we could make thousands of tags to cover all the items, all the conditions, all the verbs and then try to organize them. I'm pretty sure I made the synonym (unless someone else beat me to it).
The other would be to have a short discussion on what the tags will do for the main site. My inclination would be to look at the various envorinmental concerns and see if they are adequately searchable without needing a special tag and maybe adjusting the description of environmental / environment tag.
Great tags explain what sort of questions that tag serves to collect. Waterproof and overheating and temperature and altitude are all operating conditions that relate to the normal functioning of hardware. It might still be better for the site to have the common tag so that the people that care to answer questions about hardware usage / design can find those questions to answer.
I think we can clean up the naming so that people are less likely to get confused with environment being physical vs environment being a specific classification of variable in a specific command line context.