I don’t know if the UltraFire WF-3430 all come out of the same manufacturer or there are a bunch of them. I’ve gotten 2 @ $10 like this 1200 Lumen CREE XM-L T6 LED Waterproof Diving Flashlight which seems to be the same as the Yelz mentioned above.
They are put together with 2 halves apparently based on a seam presence. There is an obvious small ridge running down the threads and under the O-ring area. The bezel that screws on also has small ridges in the part that mates with the O-rings. These may weep under pressure and get worse as the O-ring ages.
In addition the first one had an obvious chunk out of the plastic under the top O-ring, a glaring defect. There was a kind of unevenness just off to the side of this flashing ridge. I spent the better part of a half hour cleaning this up as best possible with 1000 and then 2000 grit sandpaper. #1 survived an empty test dive to 140 feet. On the next working dive it functioned fine but had a few micro drops inside that would have been easy to overlook in an empty case. The outer silver part of the head came off when I tried to unscrew it. I epoxied it back on and it’s holding. I retired this light from diving as not trustworthy in the long run. This would not make a reliable backup unless you overhauled and checked it each dive as water would accumulate and destroy the electrics after awhile.
Number 2 is similar except does not have the gouge under the top O-ring. It still has ridges where glued together in the head and O-ring area along with some manufacturing anomalies under the O-ring area. It needs magnification to see, but it’s there. Water under pressure will find any flaw. Problem areas have been sanded down as before as well as possible. The 2nd one came with 1 extra ‘base/bottom’ square O-ring added in this model. This could provide additional protection. This new one has not been dive tested yet but appears more promising.
For not much more money and apparently a MUCH better light is this 1200 Lumen 12W Cree XM-L T6 LED 100m Diving Flashlight Torch for $20. It has a tighter spot with a smoother spill is made better, and will take a 26650 Lion. With a fresh battery it’s almost up to the standard of my $400 canister light.
There is more discussion on this light, including a breakdown (possible problems with thermal transfer), here: New cheap 26650 diving light at DX