Protecting bicycle from sea/salty water I'm using my bike to commute between my work and home (~10km). In the middle of my trip I have to use a ferry and have to put my bike on the deck. 

Within 2 weeks of the trip I could see my cassette and some nuts started to rust. Frame is aluminium and I didn't see any rusty spots on it. Since I've never used my bike this much in salty/humid environment I don't know anything how to protect it from rusting. 

Is there any grease that I should use for nuts and other open steel surfaces to prevent rust? 

Or should I just gently wash my bike every day after coming home ?
Rust on a cassette or even a chain is quite normal in this kind of surroundings is quite inevitable. Also it has little impact other than visual. More frequent wiping and greasing of the chain is one of the few remedies. Buy the cheapest cassettes so that it won't hurt your wallet too much if you have to replace more often than usual. 

What is far more insidious would be rust or corrosion on hidden places like the insides of tubes for a steel or aluminium bike. Or the kind of corrosion that will fuse parts such as bolts. So you should take bits apart more frequently and apply a thin coat of grease where metal touches metal. You should also apply spray wax to the insides of the frame.

Keep an eye on seat-post, stem and handlebar. And get stainless and coated brake and shift cables.