Getting a titanium bike may solve your rust problem, but then you have worry about those bolt cutters everytime you lock your bike outside. While security at the marina may be ok, once you ride it to work, and leaving a titanium bike locked outside is an invitation for bike thieves.

Either you get a aluminium beater, or get a folding bike that you can keep on board and when you ride to work, you can fold it and keep it under your desk.