Thanks guys, for your input!
Surely, I meant more or less expensive gear, otherwise there would be no concerns. I have heard from the people leaving on the coast that electronics like TVs, DVD players do suffer from being close to a sea. This was the reason of my initial concern. Using dehumidifier or even just a towel to protect the equipment from an extreme moisture and salt is, for sure, useful and perhaps necessary (surprisingly, this didn't come to my mind). I personally do not like to leave the gear on when they are not in use, or even leave them plugged in even if they are used everyday (after all, the safety considerations) - I have seen amplifier bare boards with a constant electric current (because of the stand-on mode) after a few years of use, in an essentially poorer conditions than the rest of the boards in the same amp.
It is interesting whether an extreme heat or an extreme humidity is worse. In fact, I didn´t know there are bare boards made without solder masks, or did you mean another type of a mask (my tube amp (Manley) does have a solder mask). But did I understand correctly that, in general, tube stuff might be more sensible to the humidity (independently of how the bare boars are made, or because of tube sockets?). In an audio system, there are two necessary components, amplifiers and speakers, one cannot avoid them. Then one wishes to make an optimal selection for a given climate conditions.
P.S. Why a class D amplifier would be more resistant to the humidity(because it keeps inside the voltage, but tube amps also)?