Summer cruise is professional training. Being on a ship, sub, with an aviation squadron for "real life" fleet training, as opposed to being bused around for tours, visits or demos to various bases for an familiarization experience.
Cruise does not equal fun in the sun with umbrella drinks in Navy lexicon.
Professional training is the catch-all phrase that covers everything other than leave a midshipman might be doing during the weeks between end of academic year and Reform of the Brigade at the start of the fall term.
There used to be 3 Blocks (1-2-3), starting right after Commissioning Day. A few years back, Block Zero came into play, starting right after exams and taking the place of intersessional leave/Commissioning Week activities. This allowed mids to add in more opportunity for internships, an additional cruise, voluntary summer school (for those who want to get ahead on their matrix, double majors, varsity athletes, potential VGEP, etc.). It also allowed for those midshipmen who, due to involuntary summer school (academically challenged) and mandatory training blocks, never got leave at all, to get a few weeks if they started in Block Zero. Before Block Zero came into being, we had one stoic midshipman who struggled for various reasons, who had 3 summers in a row taken up with summer school and summer training, no leave at all. Tuck that one away and ensure you never get into an academic slump that puts you in that peril!