The first thing you need to do is decide WHAT you want to do. Take a look at photos of the real thing. They are salty, rusty, crusty, and dirty. Decide how far you want to go with the weathering and then find ways to do it. Take it one step at a time but think about how things are going to work together at the end. If there is, for example, a chain laying on the deck then is the deck salty looking and the chain rusty? Is the deck scraped from the chain rubbing on it?
There is no cut-and-dried answer because weathering a model just depends on what you want to do and how far you want to go.