Seeing these water cans reminded me of something... I know it's not exactly HMMWV-related, but I need to get up on my soap box for a minute. I've been seeing people ask for absolutely cringe-worthy prices on heavily-used military water cans. It's really painful to watch and occasionally actually sell. People, do yourself a favor: buy these darned things manufacturer-direct. LCI is one of the military-contracted companies to manufacture these, and you can find the listing here:
https://www.buylci.com/water-can-5-gallon.html (tan or OD). Once you hit $50, the shipping is free. LCI has all kinds of things, so you could probably just add some office supplies to your order to get 2 cans up over the $50 mark. Or y'know, just order 3-4 cans. All the parts/cap threads are compatible between all brands, so you should have no trouble using whatever accessories you already have around.
Personally, I bought 4 of the OD ones a while back and can't kill them. Only trouble I've had is all the caps don't fully tighten at the same exact point, meaning the alignment of the pour spout vs vent isn't the same for every can when in use. You can fix this by adding an o-ring to change how it seals, by adding one of the self-venting spigots, or just not caring about it and paying attention to how you pour your water.
EDIT: Just did some searching around and got some O-ring specs, copy-pasted from
https://expeditionportal.com/forum/threads/lc-industries-water-can-pics.48159/page-2 (User: kd4bvp)
Where to buy:
http://www.theoringstore.com/index.php?main_page=index&cPath=367_144
What to buy:
9/16"ID X 11/16"OD X 1/16"CS (For Vent Cap)
014 B70 FDA Black Buna-N 70 O-ring (FDAB70014) $0.04 each
7/8"ID X 1 1/16"OD X 3/32"CS (For Pouring Cap)
118 B70 FDA Black Buna-N 70 O-ring (FDAB7011
$0.12 each
3 1/2"ID X 3 11/16"OD X 3/32"CS (For Large Filling Cap)
153 B70 FDA Black Buna-N 70 O-ring (FDAB70153) $0.54 each
None of my cans actually need O-rings to not leak, so you likely won't need them. Just in case you get better peace of mind, there they are.