Civilian vessels in 1/350 or larger.

There's a nice product called balsafoam. It is a little expensive, but is really easy to work with.

At the very large scale you work at, and I do mostly 1:1, vendors are the key.

Like many here, I built plastic kits before going into scratchbuilding. One of my first scratchbuilt ship was a German torpedo boat, First World War, in 1/192, since it was a handy scale. Material? Bristol and mat board. A few months later, a British fleet carrier, I think 1/768 or thereabouts, same material.

When I decided to begin building sailing craft (plus the occasional steamer), material was almost always wood, and always 1/384 (though I built a fine banana boat, the Copan, from my ancient material of choice, again, bristol). For me, it's the finished product that matters, not how you get there.

Rob