Spring breaks vary by college and school district. Typically college have spring breaks the first few weeks of March. Primary and secondary school districts usually take spring break from the end of March through mid-April at least until the week after Easter.
College spring break has more of an impact on short cruises.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013
I think you should be better in April to avoid the crowded cruises during typical spring break times. You can also tell just by the booking prices, it should be a little lower by the time you get to early/mid April.