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I'd just try to leave plenty of space open toward the bottom of the cover for ventilation so you don't end up trapping moisture that would otherwise evaporate during the day.

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.

Do you park near the ocean where you'll pick up salt spray?

I used a bike cover while I didn't have covered bike parking, and it worked pretty well. It kept the saddle and chain dry, and kept the worst of the schmutz (spring tree pollen) off the wheels and tires. You may have to tie it under the bottom bracket to keep it from blowing off.

Interesting advice to rinse it off. I wonder if the tap water really has significantly less dissolved salts than the rain water. Rain water is where the tap water comes from after all. Granted it's filtered by natural and made processes first but I don't know if low levels of dissolved salt are something they care about removing. Small amounts of chlorine are often added.

I just wonder if most of the salt exposure comes from the air rather than the rain. Rinsing the bike off occasionally would help but so would just letting it get rained on.

My experience from a non-marine climate is that leaving unpainted/untreated steel exposed to the elements will cause it to rust. A chain that is lubricated and used regularly won't rust. Many bolts/screws on your bike can be replaced with stainless steel equivalents from the hardware store for pretty cheap. Those are the things tend to rust. You can also put a light coating of oil/lubricant on them.

Carrying around and putting a cover on your bike seems like a pain to me, - but I'm lazy.

FWIW I have a brother that lives in Hawaii and rides a 40 year old Peugeot. I doubt he covers it up. I haven't seen it in awhile but I know it's got some rust on it. If I remember right he complained about having to leaving outside in a rack back in his college days. My guess is that it got scratched up and that's where the rust got started.

As a visitor there I'm always struck by how some things seem to deteriorate so much quicker in that climate while other things tend to look newer for longer.

I'm not sure how a cover would hurt anything if it's fastened in a way that the wind won't cause it to scuff up your bike.

i'm curious about this. let us know what you end up doing and if it works~ good luck OP!

The combination isn't as difficult as you make out. There's enough of a gap around each wheel to let air circulate. No, you can't dry laundry by hanging it over the top tube with the bike cover on top; however, if the bike is dry when you cover it, it won't get any wetter while covered.

And don't believe in the "breathable, waterproof" nonsense either. You'll have to get up about 5,000 feet or so from sea level in Hawaii before that has a prayer, and then only while you're wearing it and not sweating.

Another option, though a bit more expensive, is to get a bike with internal gears and enclosed chain. People in northern Europe leave their bikes outside all year in all kinds of weather and salty air and ride them for decades with little to no maintenance. Better bikes like Workcycles also use all stainless components.