According to Wikipedia, there are apparently only thirteen Nazi zombie films. This surprised me, as I'd always assumed it was quite the sub-genre (IMDB has a different list which expands the number to 48, but it also has a much wider scope), but I guess that's some cognitive bias at work there. Maybe Nazi zombie films are more memorable because, well, they're Nazi zombie films, and because they stand out to us more it seems like it's a bigger sub-genre than it actually is.
So, obviously, Zombie Lake is a Nazi zombie film, or all that Nazi zombie talk would be nothing more than a bizarre non-sequitur.
Set nominally in the 1950s, Zombie Lake opens with a young woman arriving at a lake just outside a small town in France, stripping off and, in clear defiance of the "No Swimming! - Nazi Zombies!" sign there, goes for a dip in the lake and gets pulled underwater by some Nazi zombies. (I'm not kidding about the sign, by the way - it has no words on it, but a picture of a person swimming crossed out, followed by a skull and crossbones symbol. The only way it could have been clearer is if they had added a swastika to the sign.) When the villagers find out about this, they sigh and shrug their shoulders - nothing to be done but keep avoiding the lake, they say; it's always been cursed.
In fact, while local stories might have claimed that the lake was cursed for a few hundred years, the problems only really started about ten years previously during the Second World War, when partisan villagers ambushed and killed a platoon of Nazis and threw their bodies into the lake. (We find all this out via a flashback that's conducted almost entirely without anyone speaking, effectively making it a flashback delivered via mime.) Just to further muddy the waters (hah!) however, one of the Nazis is actually a "good" Nazi - he has an affair with a local woman he saves and they end up having a daughter. Even after he's been zombified, he comes back to see his daughter, giving her a necklace that belonged to her mother and going on walks with her while the rest of his troop are eating a woman's basketball team who also decided to skinny-dip in the lake. Eventually the villagers decide they have to do something about the Nazi zombies in the lake (if nothing else it's proving to be bad for tourism) and so set a trap for them using the good Nazi's daughter as bait...
Zombie Lake's director is listed as one "J.A. Laser". In actuality, it was directed by Jean Rollin, who stepped in at the last minute after none other than Jesús Franco left the production. Unsurprisingly, a film that Franco hadn't wanted to direct turned out to be not that good at all, and so Rollin ended up using the pseudonym (a combination of his name and another, uncredited director called Julian de Laserna) rather than admit he had anything to do with the film for many years.
So as you might have guessed, Zombie Lake is not a particularly good film. It's slow, it's relatively bloodless, it has such a paucity of dialogue that you'd be forgiven for thinking it was originally supposed to be a silent film, most of the women in it are there purely for shameless exploitation (complete with underwater cameras shooting from below them as they swim completely naked) and the makeup effects look like they were produced in a primary school art class. Seriously, the extent of the zombie makeup is thick green poster paint on the zombies' faces, and maybe the occasional missing eye or head wound. Most of the time they don't even bother to extend the paint job below the chin.
Most of the deaths are bloodless as well - even when the Nazis are ambushed and shot down in a hail of bullets, only one or two actually bleed from their wounds (which probably also explains why, when the partisans throw their dead bodies into the lake, some of them can be clearly seen to start swimming). The preferred method of killing anyone swimming in the lake seems to be to just pull them underwater and drown them, and on the one or two occasions when someone is killed by the more traditional method of being bitten to death, there are no visible wounds, just blood smeared on the area.
And yet... it still can't really be considered to be the worst zombie movies out there, or even one of the worst ones I've even seen. I think that says something rather telling about me...