Disney Cruise Line captains will try to avoid hurricanes and storms by changing itineraries whenever possible. This can result in skipping or changing ports. Any Disney-booked excursions will be automatically cancelled and you won’t be charged if the ship does not stop in a port. Many private excursion companies have similar policies, but you should certainly check on cancellation policies before booking during hurricane season.
If it is not possible to avoid a storm, usually because it is expected to strike near the port on the day of embarkation or debarkation, Disney may cancel the cruise. If
DCL cancels, anything paid to DCL will be fully refunded, and passengers may be offered a discount if they book another cruise within a certain time period.
In rare cases, as jrapps described, a cruise may be extended while in progress if it will be difficult to get safely back to the debarkation port, and in very rare cases, a cruise may end at a different port than planned. In those rare cases, DCL will try to help passengers make arrangements to change flights and cancel or book hotels, but in these cases
travel insurance would be useful because DCL will not pay for anything after passengers disembark.
If a cruise is extended, that obviously causes delays in the next cruise, which would be delayed or cancelled. DCL will issue refunds for cancelled cruises and partial refunds for shortened cruises, but any costs for changed flights, additional hotel nights, and anything else not booked through DCL are up to the passengers.
So, short answer, DCL will redirect cruises as much as possible, but
trip insurance would guard against unexpected costs in the unlikely event cruises are cancelled, extended, or shortened.