In general, water only hurts the open lubricated parts, like the chain. Keep the chain and maybe derailleur lubed, and you should be OK. Rain water doesn't have salt in it, but yeah, your bike may have salt on it from the sea air, so same difference.

I'm more worried about the sun, as UV destroys just about any non metal parts. Commuting daily and leaving the bike in sun (espeically like your sun) caused my tires to develop dry rot cracks in a year. Its hard on the rest of the plastic too. So yeah, covering it with something that breaths isn't a bad idea. Me, I just park under an overhang.