The dwarven merchant clans of Barak Varr are, IIRC, notorious for playing just as hard (and by implication, dirty) as their human rivals in Marienberg.
If you can get the explosive into precisely the right place, you don't need that much easier of the explosives.
Getting the explosives beneath a ship and getting them to go off at the right time is the very very hard, requiring sophisticated detonators and the ability to control the depth of the mine. That's why we've been talking about detonators and tethers, and the difficulties involved in making and deploying them. You basically need early to mid twentieth century technology to make something that will work automatically, and mid to late nineteenth century tech to make something that can be triggered by a remote command for this kind of mine. Otherwise you end up with something that blows up next to the ship; not beneath the ship, which then needs the vastly more powerful explosives we moved on to talk about to sink the ship.
This is a generally renaissance setting. I don't believe Marienberg has nineteenth century technology.
There's a third scenario, though even more difficult - a diver intercepting the boat and manually attaching the bomb to the hull. That'd also produce sufficient yield, except:
-The required bomb is FAR heavier than what a human could swim with. As in out-masses the diver. Not impossible for a vampire.
-The water of full of piranha
-The dwarves are not blind and attaching a bomb is not very quiet or a normal sound.
Why is hit rate correlated to size?
Cross sectional size templates, The bigger the target the easier to hit even if your trajectory is not perfectly straight due to water currents. Remember, small errors in angle produce large deviations when accelerated for torpedoes.
There's two different types of technological advancement. One is incremental inventions that can't happen in very different orders or time frames, requiring specific materials, advancements in industry and prior tools and such. The other are advancements that have had all necessary tools available for quite some time, but had no one think of it yet. This particular one seems like the latter while stealth bombers and surface to air missiles are decidedly in the former.
This really is not the latter.
Its like saying you could use a musket to shoot down a low flying plane.
Your tool, if perfectly employed by the best possible user in the best possible way, cannot hit the target by any means other than pure blind luck, it does not have the necessary precision.
It requires specific innovations in mechanical engineering, material sciences and chemistry to create the
possibility of performing such a hit, then you need further innovations to make it anything other than a one off superweapon because its too expensive in skilled manpower to make every single component.
THEN you can develop the tactics needed to use it effectively.
The key developments:
-Explosive yield - Gunpowder is too low. It COULD theoretically work on a fixed target given unlimited setup time, but its bulk per unit bang, its sensitivity to water, its consistency and how said consistency would deviate the larger the quantity employed. You'd be better off with multiple smaller charges
-Delivery mechanism - The explosive must be brought up to the target undetected. The larger the charge the more difficult this is. Several hundred kilos of gunpowder would be very difficult to deliver. A few kilos of warpstone would be easier to deliver of course.
-Trigger mechanism - The window for an effective(disregarding ideal) strike is less than five minutes, you're dealing with a fastmoving river and a fast moving ship. The trigger must either be manually operated, or be able to detect the ship, or be divination level perfectly timed to achieve this. Noting that the river is a very noisy environment which could prematurely engage a large variety of possible triggers. Enchanted, runic, or engineering masterworks may achieve this without the necessary tech base, but those are effort intensive, unreliably created and unique configurations, you wouldn't use it on something like this.
Everything is much easier if you use warpstone for everything though
