We have established that the list of individuals capable of this attack isn't large, and this changes the capability ranges.
Marienburg could do it, but it would take a substantial commitment of skill and resources. Said commitment would be tracable and worthy of special attention. A degree of attention which also would mean they should know who they were hitting, it would not make any sense if they invested so much resources to hitting an ironclad monitor with a sea mine and wind up blasting the wrong target. And the Okral have literally no reason to be involved in the canal, something which the dwarves on their council would have advised.
The Skaven could do it, are known to be previously active in the region, and would benefit from starting up a conflict between the Karaz Ankor and Marienburg, who'd be the first suspect.
The Chaos Dwarves could do it, but its the wrong body of water and they're largely uninvolved in these parts.
The Greenskins, Bretonnia, Border Princes, etc cannot do it.
The Empire could do it, but at a resource cost that largely rules out a Lone Gunman scenario, which would require someone to go against both state religion and national alliances with a powerful organization backing it.
The Vampires could do it but again, nontrivial effort, WHY would they do it other than be evil?