…Authorized Necromancy sometimes ain’t pretty, friend.
First off: yes, it’s just as awful as it sounds. Take one freshly dead seal — what? Yeah, sure, OK, they died of natural causes, with their grandpups surrounding them. Whatever lets you sleep at night — then open it up and emplace a magical homing receiver in the head, and a mildly enchanted exhaust pipe in the, err, rear. Stuff all the soft bits that you cut away to make room for the equipment in the pipe. Also stuff in all the soft bits that you cut away to make room for whatever explosive impact powder you happen have to hand (in waterproof containers, of course). Now zombify your seal.
Turns out, if you stick the receiver in the right part of the brain, the zombie seal will bliss out — as much as zombies ever ‘bliss out’ — and follow the ‘sound’ of the bell that only it can hear. Said bell is, of course, a specific magical homing transmitter, which is usually installed in a crossbow bolt and fired at whatever naval target you want to blow up. Get the bolt anywhere near the waterline and the Seal Torpedo can usually manage to impact the hull hard enough to set off the explosives. Particularly if you think to turn on the exhaust pipe: several weeks to months of rotting goop tends to produce a reasonable amount of noxious, yet flammable gas. Note, also: Seal Torpedoes can in fact steer themselves.
So why did they do this? Because your average species invents the sailing ship before they invented self-propelled torpedoes, of course. Somebody in some navy told some magicians somewhere that they needed a way to sink ships magically that didn’t involve having a full-time trained specialist on hand, and there apparently was a Authorized Necromancer in that particular crowd who was desperately trying to show how much of a well-socialized team player he was. And since the only things getting used up were, well, animals; behold! Seal Torpedoes.
What? No, using seal pups is counterproductive. The entire idea was to have an Undead creature that could actually move quickly on its own. Besides, the pups are all needed for the breeding program. Which would be the breeding program that will happily pay top government dollar for any shipment of live seals that get brought in for, ah, ‘processing.’ Easy money, really…