If you want to sail from Galveston, I would suggest Royal Caribbean over Carnival if you do not want an invasive party atmosphere, but be aware that during spring break, there are going to be lots of college students looking to party everywhere (including most cruise lines)
My wife and I did the Splendour of the Seas out of Galveston earlier this year and did a MEGA review of her and the itinerary at;
http://www.cruisereviews.com/RoyalCa...ftheSeas14.htm
This should give you an idea of how it is.
Have a great cruise.
Tom