Water mines
Mines, nets and fireships, for fighting on the rivers.
Mine Workshop Makes water mines from explosives and metal blocks. 
Mine Slipway Releases floating mines that drift with the current and explode against the first building they hit. 
Moored Mine A mine anchored in the water. Goes off when a boat or a drifting mine comes close. 
Fireship Dock Launches boats packed with explosives that sail up the river and blow up against the first building or mine they hit. 
Mine Net Floats in the river and catches drifting mines. Lay several end to end to make a barrier. 
Net Winch Its worker rows out to the linked nets and brings back caught mines for reuse.