The box for Shock Waves proclaims it to be the best of the Nazi zombie movies. This may very well be true, based on the only other Nazi zombie movie that I've seen, Zombie Lake, but it's not the kind of thing you brag about. Still, this wasn't that bad a movie.
The movie opens with a woman being rescued from a small boat adrift at sea. After being revived, she begins to recount her story. She and several other passengers had been on a cruise aboard a run-down ship, run by Captain Ben (John Carradine, who seems to be unable to stay out of these movies). One day the Sun started acting freaky, and then that night they ran afoul of a ghost ship and ended up running aground. They were trapped on a nearby island, only to discover that they were in danger from a division of Nazi zombies who had been lurking around underwater since WW II. An old SS commander (Peter Cushing) is living on the island, to add some further tension to the plot.
The movie started out building tension nicely, and even though not many zombies appeared on screen for quite a while, which normally annoys me, their presence was strongly implied, so that was OK. Then the movie seems to lose steam, and it also starts to get choppy. It seems at times that there was dialog that we missed, or some kind of connecting scene that would explain things it seems we're supposed to know, since otherwise the actions of the characters sometimes come out of the blue. Zombies pop out of the water occasionally to kill someone, and once you realize that there's not much more to the movie than that, and knowing the ending, you might lose some interest. Still, it was entertaining.