They're far from "water tight".
With a quick dunk in water, or constant rain on a loaded firearm, and standard ammunition (including reloads) should not have a problem.
...But it should be considered water "resistant", not "water tight".
Almost ten years ago, now, I started testing water incursion into military ammunition, factory SD ammo, factory 'blasting' ammo, and reloads exposed to submersion in water.
Yea, yea... most people aren't going to be storing their ammunition under water, but I wanted to know how each type of sealant (or lack thereof) performed under very undesirable circumstances.
.380 Auto, 9x19mm, and .30-06, submerged in 6" of water.
I had control cartridges, and dunked the rest. Samples were removed in pairs: One cartridge to pull down to check for moisture, and one cartridge to attempt to fire. The cartridges were fired once a week, after being stored in a sealed bag (case thoroughly dried on the outside, then bagged).
Samples were removed every few hours at first, then every day for a few days, then once a week for six weeks, and then once a month for three months. ...And then I forgot about the test for about a year.
A short(ish) summary of the results, from memory:
ALL unsealed ammo had water incursion within 24 hours. Reloads were even worse (likely due to carbon residue in the necks and primer pockets).
Reloads sealed with Markron primer sealer did slightly better than nail polish, but both were wet by the end of the first week.
Most sealed factory ammo had evidence of moisture in the powder by about week three (9mm Speer Gold Dots were the primary contender here).
With two exceptions,
all loads had soaking-wet powder by six weeks in.
The two exceptions: 10+ year-old Winchester SXT .380 Auto SD load (the "not a Black Talon" load from the '90s -- now PDX1, or whatever it is). And some old LC 67 .30-06 that had been pitch-sealed.
The SXT and vietnam .30-06 was, on average, bone dry after all others failed, even after being submerged for a total of almost 18 months.
But, even then... Both the SXT ammo and the surplus did have cases that had wet powder inside them at some point during the testing.
No matter how well the ammunition is sealed, there's always a weak link that doesn't quite make the cut. Will that weak link be the cartridge in the chamber when you need it the most?...
(I was being a bit facetious with that last statement. Unless you store your ammunition under water, it won't really matter.)
My takeaway was that if your ammo will be getting wet, it needs to be sealed (a good cleaning before applying Markron being the assumption for reloads). And if your ammo might be exposed to water for a prolonged period of time (such as with water/snow sitting on the cases heads all day while in a cartridge carrier/belt), you'd better figure out how to pitch-seal.
But, of course, it's well within reason to also reach the conclusion that it doesn't really matter if you aren't submerging...