The ships also have at least one rescue boat, which is capable of marshalling and towing liferafts and performing rescue of persons in the water.
We've only sailed in the Bahamas & Caribbean, but I have noticed, especially at night when you can better see the lights of the other ships around, that the ships largely travel in or close to what looked like established corridors (the ships actually form kind of lines, which when there are enough close together you can make out) and there are actually a surprising number of other ships around; they will be the first help that comes should it be needed.
Ideally of course the cruise ship itself serves as its own large lifeboat-of-sorts, as passengers in most circumstances are safer on board the ship then they would be evacuating it -- they speak to this during the Muster Drill. Passenger ships built in the last decade or so are actually required to meet on board "safe area" requirements for this reason.
SW