Wide-spread gastro-intestinal illnesses -
google "cruise" and "norovirus"
http://www.cdc.gov/nceh/vsp/surv/gilist.htm
It is important to distinguish among cruise outcomes -
I'd rather experience a 24-36 hour power outage than
a fire that resulted in passengers at sea in lifeboats,
but there appear to be some narratives that conflate
"our honeymoon was not what I had dreamed of" with
"captured, burned, sunk, and destroyed" without
any points on a scale in between.
Third party advice I'd heard on cruising:
avoid the very cheapest lines
choose an operator with a generally nordic background
(not flag/registration, more company origin).