Norwegian has come 7 night cruises to the Bahamas. Some have one night in Bermuda.
If you can affort a 8-night cruise there are some that do the Caribbean on Carnival and Norwegian that includes St. Thomas, Puerto Rico, the Bahamas. Princess has a 9-night to the Caribbean also.
Check their cruise sites for the month of August to see where these ships go.
There's also Canada/New England on Carnival but they are only 4 or 5 nights.
Hope this helps get you started in searching these lines to see what they offer.
2008 Liberty OTS (Caribbean)
2007 Summit (Alaska)
2006 Century, Infinity (Panama Canal), Summit (Hawaii)
2005 Millenium, Voyager of the Seas, Jewel of the Seas B2B
2004 Explorer of the Seas, Constellation (Canada/New England)
2003 Horizon (Bermuda), Summit
2002 Explorer of the Seas, Brilliance of the Seas (Caribbean)