I don't know of any major cruise lines that do "summer employment". Contracts are usually six months or longer.
I have copied this thread to our Crewmembers area. Perhaps it will increase your chances of getting more information.
Take care and good luck,
Cruisemates Community Leader/Moderator
Upcoming Cruise(s) Carnival Liberty: May 31, 2014. The Anniversary Getaway.
"There is a great difference between being well traveled and just having been to many places." ~Me
"Fear is the assassin of dreams." ~Me