To my knowledge summer cruises are not optional at all. If you are on scholarship or otherwise contracted you will be required to attend a summer cruise, although there is some flexibility as to when. If you are a college programmer (not contracted) you cannot attend a summer cruise.
Here's something of an example. My DS had a summer job at the college which would definitely come up against his standard summer cruise for that year (MO Mountain Warfare). In an effort to continue in the job until it ended in late June he requested a change in his cruise to actually serve on a ship.... somewhat odd for a Marine Option. The change was permitted. Unfortunately for him, when his orders came through, it actually interfered more with the summer job than Mountain Warfare would have. Great cruise though... LSD-48 from Norfolk to San Diego via Panama Canal. He worked with the weapons team and was on a ship he actually might serve on one day.
Some summer cruises are earlier, some are later. You can REQUEST a time frame, keeping in mind that you are ultimately under the orders of the Secretary of the Navy.