Well, for starters, unlike armor, you have to be careful weathering most steel-hulled ships, and constantly think "less is more" else your creation will turn out like a pointy-ended rust lump - unless you are specifically out to create a rustbucket merchantman or something similar, like this well-used Liberty ship:
All of the rusting on that model was done with the drybrush method, taking a small amount of paint on the end of the brush, removing most of it by blotting against a paper towel, and then applying the remaining paint to the model with a single stroke from the source of the "rust" to the end point. I used 3 or 4 different shades of paint to avoid a monochromatic look, drybrushing some shades over others.
The other popular method is to use powdered pastel chalks (not oil pastels), which is what I suspect was used on this fine-looking model. You buy various shades of orange, brown, black and gray, rub one on a piece of paper until you get a small pile of the chalk dust, then apply with an old paintbrush and blow off the excess. This method is very good at highlighting cracks or crevices, and for creating more realistic exhaust effects, as seen on this G4M Betty bomber:
With either method, remember that, in general, warships were kept pretty clean (chipping rust and painting is a great way to keep idle hands busy) so unless you are modeling a convoy escort that had been at sea in the North Atlantic all winter or something similar, it won't be an allover rust color.