In the Korean and Vietnam Wars there was no submarine threat, so the days of packing 10,000 soldiers/marines onto a stripped down ocean liner were over. Correct me if I'm wrong, but I recall the sister ship of the Titanic was sunk as a hospital ship during WWI (Brittanic?). I think another sister ship was used as a fast troopship (Olympic?).
LOL This time, I'll actually give you the link!