Cruise lines do have a cut off a few days ahead now since the manifest has to be sent to Homeland Security. Some lines do let past passengers book late since they have your information on file already as a "member" and can push that through.
Cruise lines also sell some space as "waitlist" that can clear up to the last minute pending a cancellation, but there's always the possibility that no space clears too. Got to be really flexible, and depending on the space that clears it's not always as low as $300.00!
Large agencies also buy blocks of rooms. With some contracts those cabins are owned outright and close to sailing they will try to sell them for any price. Most contracts specify that unsold cabins will revert back to the cruise line XXX days before the sailing, usually with enough time for the cruise line to sell them itself without steep discounts. Of course if they are not selling the cruise line will discount those further and also clear the waitlist cabins to sail too.
That Boston-Bermuda itinerary is usually lower priced anyway from when it starts in May to late June when schools get out, then the prices spike for the peak summer season.