CBC Is reporting 2 containers were lost in the water at Halterm.
ZIM Vancouver and ZIM Haifa are both tied up at Pier 41/42. the most likely scenario is a container was swung by a crane  into the stack aboard ship, knocking 2 into the water.

Update:

I’m told it happened Early into the Night Shift, and shipfax stated it was 2 20foot containers. these can be handled as a pair, so its possible the spreader on the crane failed.