How humidity levels affect piano strings is one of the most important things to consider in maintaining the instrument. The strings are polished bare steel. Think of what would happen to your car, sitting outside, with no paint to protect the sheet metal. It would rust out within a very few years. Now, think about the strings in a grand piano, sitting in an ocean front home or resort. Most of them will need to be restrung in about 10 to 12 years, if not sooner. That is not very long when you consider that if protected properly, they could last well over 75 years! Not only will the high RH rust the strings, but the salt in the air will be just as destructive to both the strings and the tuning pins. The result would be poor over-all sound and eventually broken strings.
Note: The picture below is of a piano that set in an open home right on a south Maui beach.