USMC are using Trijicon dot sights of some description. Trijicon's are pretty tough and dual powered to gather available light or use tritium inserts, depending on the operating conditions, to provide an aiming point. Some are fairly low powered. Might look into them.
However Aimpoint makes several models of non-magnified dot sights that claim up to 50,000 hours of battery life. That's a long time, even if you forget to turn them off for a year or so, and the Army has been using them for several years now.
None of those are particularly cheap, but have the dual advantage of being tough and work well.
"There it is"
LOAD AND BE READY!