One technique I've found useful in adding a weathered/rusty appearance to non-steel boats is to drybrush
very light streaks of rust colour downwards from fittings, rivets, windows etc. and anything else that might
be made of steel on the hull and superstructure, the streaks getting fainter and narrower as they get further away
from the rusting part.
I used this method on my Vosper MTB (wooden hull) and Asheville-class gunboat (aluminium hull AFAIK).
Another useful method is to lightly dry-brush the edges and raised areas of the boat with a slightly lighter or darker shade of the base colour. This highlights the details and gives a less "flat" appearance without actually representing weathering as such.
On an aluminium boat, a small amount of metallic scuffing/chipping on edges and corners might also work well.