If you are thinking of going on a cruise in September, I recommend that you find out if your destination is in high or low season, to know if it is going to be very crowded or what prices you are going to find.
To give you an example, in the Mediterranean you can find very good prices in September and October, on new boats. The only thing is that it can rain on you and your cruise, or mini-cruise, becomes a trip inside the ship. I'm talking about a average price of 500 euros per person on a 7-night crossing sailing through Spanish, French and Italian waters.
If you decide to go further and have a romantic journey around Bermuda, remember that at this time it is also low season, so luckily you are going to find very interesting prices. The temperature ranges between 29 and 24 degrees, and there is an average probability of 10 days of rain in the month. I have found several Cruises for about 450 euros, 8 days long, yes, the ticket does not include the price of the plane.
Remember that in the Caribbean September is hurricane season. This does not mean that you cannot travel or that it is dangerous, it is just that the captain will likely decide to change course. Here You have some tips if you decide to travel in hurricane or tropical storm season.
Curiously, the cruises that take you to Canada will be in their peak season, and it is that the autumn coinciding with the change of color of the trees is a wonderful spectacle, without the harshness of the winter climate. In September the average is 16 degrees in the second fortnight.
As for river cruises don't have as much price swing, although September is already considered low season. In my opinion it is the best time to navigate through the center of Europe, or to get to beautiful Porto, for example.