Imagine going to a steakhouse, your mouth watering for a juicy filet mignon, only to find out all they can serve is a burger.
Oceania can't control the weather, but to an extent they can control their alternative ports of call. Instead of Nassau, how about Charleston, SC? Jacksonville, FL for historic St. Augustine? Turks and Caicos?
Nassau is a lame substitute for Bermuda. If you are just booking for the cruise and don't care where the boat goes, then it shouldn't matter, but if you booked wanting to go to Bermuda, then you're probably not going to be happy with the Bahamas. Bermuda is a chic and elegant island outpost and many people think Nassau is the armpit of the Caribbean. I would be disappointed.