If you are thinking of doing some XP10 customizing for your trip, a strap over the hatch could be a good idea -- maybe not as critical if most of your gear is on a raft. If you scroll half way down the linked page (self-support GC trip), it looks like there is such a strap on the yellow boat.
Liquid Logic Remix XP10 review by Ted Keyes
The XP10 hatch has a tab at the stern end that fits under a slot in the hatch coaming. The hatch then closes pretty well when you push the hatch down all the way. There is also a "tab" on the front of the hatch. A thin rubber strip attached to the front hatch coming can be pulled over that tab for some extra security (sorry, no photo). That small rubber strip on the XP10 I have rented has disconnected on one side (probably repairable) -- not sure if this is due to abuse by renters/students or if this is a weak link in the chain.