Interesting. oneway isn't mentioned on https://wiki.openstreetmap.org/wiki/Key:waterway or https://wiki.openstreetmap.org/wiki/Tag:waterway%3Dflowline.

However while these are tidal bays and the current changes based on the tide, downstream is still towards the ocean and the way should be drawn that way.

What would oneway=yes/no even mean in this context that's not already covered by tidal=yes/no?