I do not see much new opportunity for new American flag cruise ships. Unless, foreign flag cruise ships are significantly restricted In the US cruise market.
As it is, Hawaii and River Cruises are about the only possibilities for American flag cruise ship growth. The Hawaii cruise market appears to be stuck at just a couple ships, maybe that's the extent of that market. American River cruises could be developed, but our rivers, and most US river cities, are not all that nice, and are unlikely to become a large cruise market. We could never compete with what European rivers have to offer.
Alaska could support a significant US flag cruise business, but only if foreign flag ships are severely restricted, e.g. size limits, limited number of stops allowed between foreign ports, visa restrictions, etc.
As far as OSV conversions to cruise ships go, I'm not sure that is practical. The market trend seems to be toward ever larger and ever fancier ships. OSVs are too small to compete against the larger and more stable ships. No matter how cheap existing OSVs may become, the cost of conversion and operating cost would seem to be too high per passenger carried.