location, location, location... river is shallow but why?
Maybe once there was a bridge that has now collapsed blocking part of the channel, part of the bridge could be used as death from above platform and ghouls attack from it.
Then you have the river bank, how far away from the barges, are they cliffs or just a smooth bank into the water, any ghoul reinforcements will be coming from there. Any landmarks on land, old battle site, ruins, etc?
Now, the nature of the river, what is underwater, you can have drop offs, pits, traps like shifting logs or stones, mud, etc.
Thrown in a Shambling Mound as part of the river wreckage or a mimic or two as logs or cargo lost.