The park has little shade, and by 10 a.m. it can be packed with people, hot and humid - I would expect that in July. The best plan is to be there early, and then tour the park and be out of there by 10 a.m. July will be hot, humid, and very busy - I would avoid going in the afternoon.
The park is not an all-day event, it basically has a museum, bookstore, and a video to watch, and the launch out to the memorial. If you get there early, you can see everything in 2-3 hours, easily.
Note that security is very high and you cannot take ANYTHING into the park except what's in your pockets, and a hand held camera. You cannot take purses, backpacks, diaper bags, or any kind of bag. There is a place in the parking lot that is run buy an outside contractor where you can check your bags, but when I was there, they stored things in open bins, so it wasn't secure. A better plan would be to put cash and a credit card in your pocket, and store other valuables in your room safe.
Security Measures
Strict security measures prohibit purses, handbags, fanny packs, backpacks, camera bags, diaper bags, luggage and/or other items that offer concealment. Visitors may bring cameras, cell phones, and wallets, although since no bags are allowed, you are encouraged to pack lightly.
A storage facility operated by the USS Bowfin Submarine Museum & Park is available in the immediate area of the entrance to the Pearl Harbor Visitor Center. There is a nominal storage fee of $3 per bag. The storage facility is in operation daily 6:30 a.m. to 5:30 p.m. Visitors may use the same bag storage and parking stall for visits to all Pearl Harbor Historic Sites.
These security measures are strictly enforced at all visitor destinations on Pearl Harbor. Although federal law enforcement officers and security bike patrols monitor the parking lots, please do not leave items of value in your car.
Here is the visitors guide with more info. -
http://www.nps.gov/valr/planyourvisit/feesandreservations.htm