I presume that rakurai fish are a native Safeholdian species rather than renamed Terran torpedo rays brought to Safehold by Operation Ark. However, they seem to occupy a similar ecological niche.
This leads to me wonder why the Jihad side didn't at the informal, don't let an inquisitor overhear you, level refer to sea bombs as rakurai, just as early sea mines on Terra were often called torpedoes. (What we now call simply "torpedoes" were originally called "automotive torpedoes" to differentiate them from "floating torpedoes" and "spar torpedoes".)
Speaking of which, something like the early Whitehead torpedo, powered by compressed air, seems to be well within the proscriptions, but the relatively simple guidance mechanisms used by early torpedoes to maintain a steady course and depth are probably complicated enough that only Charis would be able to develop them, except they wouldn't put them into service as it's a device that provides more benefits to smaller navies.