It depends. Usually the summer starts out very lax (youngsters leaving the yard in civvies, weekday liberty, overnights for all) until someone does something stupid and gets caught (underage drinking, being seen in civvies by the wrong person, repeated missing taps) and it becomes more in line with what goes on during the ac year but still much more relaxed. Mando summer school has formations, CDOs, and a more formal organization. Not sure about voluntary.
Watch exists, but in a slightly different form. Until plebe summer starts, Main-O, colors, and the other watches (I'm pretty sure there's no company level watches during the summer) are stood by NASP (Naval Academy Summer Programs, i.e. the people there for YPs, summer school, or sailing), restrictees, and Summer Seminar, which has its own watchbill as well. During plebe summer, the OOW, MOOW, MainO, Colors, and misc. are stood either completely or primarily by officers assigned to plebe summer, detailers and plebes.
Clear as mud?