Sienne Estuary Depth?

[Rex]

I'm trying to make a mission, and in part of it, the Allies try to cross the Seine just south of Le Havre in Higgins Boats.  Problem is, it won't work because  the estuary is so shallow ... too shallow even for Higgins Boats.    The thing is, I thought those boats only had a draft of like, 4 feet?

I know nothing of the water navigation in that area, Is the Sienne estuary really that shallow?