Depending on how "close" the ocean is, you may also want to consider wrapping the cases in airtight plastic.

The humidity's not the major factor I'd worry about if the house is RIGHT up against the shoreline. The drying factor of the salt would be. A drummer for a group I was with stayed a weekend at a "shoreline" hotel, and the head of his bodrhan dried out with visible salt crystals when he left it outside for a few hours. Skin was ruined.

Something to consider. Oh yeah - and the obvious, loosen the strings.

M