Looking good so far! I wonder if you saw this pic in your researches? It's from the Naval Armed Guard site:
http://www.armed-guard.com/la040c.jpg
As for inexpensive landing craft, I don't know what resin ones would cost, but for about $17 you can get a Revell USS Montrose kit which includes about 18 small landing craft in 1/375 scale. You could use some of them for deck cargo.Then you can just throw the rest of the kit in the parts box.
Lighter type vessels wouldn't be too hard to scratch build, maybe a fuel barge? How about putting in some floating garbage and such. Harbors were none too clean in those pre EPA days. :)
Fred