Blame this.
Skullcrabs
These creatures are why most civilized realms frown on the gratuitous use of zombification as a means of powering galleys. It admittedly seemed logical enough at the time: undead rowers don’t tire, don’t need food or water, don’t try to revolt at inconvenient times like the middle of a sea battle, and don’t really ‘die.’ They just break, and when one does that you can just toss it over the side.
And that was the problem. The combination of sea water, undead flesh, and the peculiar metabolism of certain crustaceans has created the Skullcrab: it superficially resembles a allarming-looking purple-black hermit crab that uses a skull for a shell, but it’s actually a lot nastier than that. Basically, Skullcrabs are unholy (in the academic sense of the term) hybrid creatures that have a continuous hunger for meat and the inherent intelligence of baboons. Individual Skullcrabs can be nasty surprises for a human, although not usually fatal ones; but when you get about a hundred of the things in one place, it can be bad. They’re organized enough to operate via pack tactics, smart enough to swarm sentries first, and know enough about technology to cut ropes and wreck equipment.
As for a Skullcrab’s lifespan… it’s unknown, but it’s at least a few decades. Or possibly centuries. When they run out of food a Skullcrab will simply go dormant; they certainly can’t be starved to death. Indeed, the only really reliable way to put down a Skullcrab is to, well, crush the skull.
Fortunately, they absolutely cannot reproduce on their own. To make new Skullcrabs you need to take a reanimated corpse that’s been submerged in seawater after being surrounded by organized death and destruction, then feed the corpse to a hermit crab. Said crab will eventually take over the skull as its new shell, thus completing the Skullcrab transformation process. And that process will not happen en masse if people remember to not use zombies as galley rowers.
Also: do not eat Skullcrabs. That one’s just on basic principles, though. Eating revenant flesh never ends well.