InsureMyTrip (an affiliate site).  I recommend them on the basis of their broad selection and excellent prices.  You can compare fine print from different insurers, compare prices, and purchase your policy online.  When I purchased my coverage, InsureMyTrip (an affiliate site) had the most competitive prices.
Insurance Providers

The following is a list of third-party travel and cruise insurance providers.  "Third-party" insurance is a term used to refer to insurance that is not sold or underwritten by the cruise line or an affiliate.  Third-party insurance is the ONLY way to guard against financial default (bankruptcy) by a cruise line.  However, some cruise lines are excluded by certain policies.  Make sure your cruise line is included before purchasing your insurance.
Of this list, the only one that I have purchased from is
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With the exception of InsureMyTrip, the following Agents and Insurers are presented for reference only; I have not used, and do not endorse them.  This list was originally compiled by John Cook, and reposted on rec.travel.cruises by George Leppla.
Agent
Insurer
CSA
Commercial Union
Globalcare
Commercial Union
Travelsafe
Commercial Union
Travel Insurance Services
CNA Insurance
Universal Travel
CNA Insurance
Travel Guard
ACE
Travel Insured
The Travelers
Access America
BCS
Unicard Travel
National Accident
Travelex
National Accident
InsureMyTrip
Access America
Travel Insured
HTH
MultiNational
TravelGuard
TravelSafe
Unicard
Universal