From the Figurehead range:
1BL1 Otranto, Otway, Orama (A,T)- troopship, hospital ship, AMC – excellent as a generic small liner/large transport
1BL2 Campania – good large troopship
2LB7 Circassia – Should make a fine IJN troopship
2JM2 Japanese war emergency tanker
2UM2 Auxilliary Oiler – should do as an alternate tanker
1BM1 Large Tramps – generic
1BM2 Medium Tramps – generic
1BM3 Small Tramps – generic
2BM4 Large Freighters – generic
* all 4 models are pretty typical steel hulled freighter common from 1910-1945
1JM1 Colliers
1UM1 Colliers
* You'll have to cut off a few kingposts but they make good larger generic merchants as well
1XM2 Sinking Tramps
1XM3 Sinking Colliers & Oilers
*just for fun, really neat models