To add to my previous response:

A large aquatic mammal at Montlake could be a beaver, nutria,
river otter, or (maybe) muskrat. If you are certain it did not
have a flat tail and it was too big for muskrat, then nutria or
river otter are most likely. You imply you got a good look at
the face. A nutria, a rodent, has the face of, er, well.., a rodent,
while otters look like, er, well..., mustelids. I noticed you work in
the library. Try to find pictures of the two to compare.

Kelly