It's tough work deleting stuff and adding nodes!

Overlapping water areas, you say?

relation/19390396
relation/13663366

Thanks for catching and fixing this!

I have requested help cleaning up this import in a thread on the community forum: https://community.openstreetmap.org/t/help-with-cleaning-up-import-in-sarnia/141866

Oops, incomplete. Continued in changeset/179004555 and I also added commented with reasoning there