From my own experience, some IEMs are susceptible to liquid damage, but most aren't, at least to a realistic degree. You hear stories of people damaging their brand new $800 pair with a little liquid but I've used more than 10 different pairs, all from different manufacturers in fairly heavy rain for relatively short periods (30 minutes or so) and they've been unaffected. Heck, I once put a pair of SE215s in the wash by accident and they still work today, albeit they are 10 years old and have had a little tlc over the years lol. Some come with IP ratings, so if you want to be on the safe side I'd look for a pair with a similar sound profile to what you're looking for with that added protection if you do need to use them all day in bad weather.