More likely the seal under the water pump.
The "gear housing cover" has 3 seals.
The big o-ring for the base of the pump is about the only place that's connected to the water passages.
Find a parts diagram for the lower unit and look at the water pump base.
Drop the lower, remove the oil and water pump.
Spray with soapy water, then add a bit of compressed air.
It should bubble where the leak is.