Waterproofing is a thing and magic simplifies the trigger mechanism substantially, all you need is a spell to detect when something touches the mine hooked up to a spell that ignites a small flame in the gunpowder compartment.
Bolded for emphasis.
Master Wizard Mathilde Weber with heroic Learning has expressed that one of the hardest part of a spell is target selection. Look to the Fog Path, which seeks a specific critieria and took a lot of reasoning to work out.
The only trigger mechanisms that'd be working in those circumstances(waterproofed explosives of sufficient yield to penetrate the hull) are actually a very narrow list:
-Chamon enchantment - Most Chamon users in the Old World are Imperial, and certainly you could cobble together a Chamon spell to detect metal and then stoke a fire, they have effects for that. It'd be high end Magister work however.
-High Magic - Only the Elves or Slaan, and in both cases, non trivially.
-Warpstone contact explosive - Rely on Dhar wanting to explode. Yield addressed by warpstone charge
-Bound Daemon/related being - Chaos Dwarf or a skilled Magister with the right secrets to make it work.
-Timed fuse - If you want accurate time, clockwork fuse in a wax sealed container of air and powder, with a flint. If you want cheap, slow burning fuse and drop it into the water just before the monitor reaches.
-Mundane fuse - Ancient china explored this, at this tech level you're looking at using a candle securely mounted to a base, which would topple onto and ignite the charge when knocked over. However, regular choppy water can set it off prematurely.
-Magnetic fuse - You're looking at a Master Imperial Engineer, Dwarf Radical or Skaven at work here, but its possible to improvise a magnetic trigger using a binary explosive. It'd be a masterpiece.
And the thing is...every single one of the above would leave Windsight visible evidence.