abrams, think everyone has holiday on their minds, most especially midshipmen who just wrapped exams last week and are home. Appreciate your patience. My response is based on my BattO time and interaction with the 16 midshipmen over all USNA classes we sponsor. We see them all summer long in between training blocks, and have watched the process unfold each year of their requests and eventual assigments during the summer.
Good question. In general, there are certain mandatory summer training blocks you must fulfill, based on class, and there are options for blocks designed to help you shop around for your eventual service selection or develop leadership skills (NASS and plebe detail). There are also some fun elective things to try for, such as the internship with the Japanese space program two of my 2/C sponsor daughers did last summer. Somewhere in there, you squeeze in some leave, assuming you aren't in involuntary summer school (you messed up during the academic year), voluntary summer school (you're trying to get ahead on your matrix or are a varsity athlete trying to achieve a lighter acyear load), or, horror of horrors, on restriction with no leave at all.
So, based on the guidelines for your class, you might ask for a surface cruise in Pearl Harbor, or a sub cruise out of Norfolk, or an aviation cruise out of Jacksonville, and depending on the "needs of the Navy," (a phrase that will become a big part of your life), Fleet operational commitments and available funding (flying people to HI is not cheap), your wishes may become true. Or, the aviation part is fulfilled, but you're not at the geographic location on your wish list.
As to how the feeds into summer training assignments from USNA and NROTC get meshed together, that's handled at higher levels, with much coordination with Fleet operational staffs between ROTC hq, ROTC units and USNA. Ships are told they are getting x midshipmen at various times over the summer, and it will be a mix of USNA and NROTC midshipmen. With so many moving parts to coordinate (thousands of midshipmen + operational unit skeds + real world events), who gets what first based on their commissioning source is pretty much invisible on the decision scale.
Clear as mud now, right? Semper Gumby is the key!