If you decide to do the boat tour (I wouldn't as 5 hours isn't nearly enough time), you'll be hot/sweaty and that's how you'll get to be for the next 17 hours of your travels. You wouldn't have enough time to grab a shower in the C Concourse lounges most likely as you'll have used up that time. Also, ALL carryon luggage is checked by CHUBB security agents prior to boarding USA direct flights, so there is a line before you go downstairs to the boarding area (this might also work to your advantage regarding a shower as the flight will probably not leave on time at 7:40pm. We left at 8:30 two weeks ago). Keep this in mind (also for the benefit of the person sitting next to you).