I dove near Pier 69 one time some years ago. There was a lot of construction junk near and under the pier and lots of mud away from it.
I didn't see any two-headed fish, but I felt like I should have. I wouldn't do it again -- I'm not that fond of urban diving, particularly that close to the mouth of the Duwamish and in a bay that's seen 100 years of post-industrial waste.
p.s. I was at the new Myrtle Edwards beach last December for a press conference with the governor. We aborted our dive plans because the wind was howling, the waves were big, and the debris being tossed in the surf was huge and dangerous -- so we went to the art museum. But in a moment of unexplainable insanity I waded into the water briefly to stick my camera in it to see if I could film anything interesting. Mike Racine was watching my back because of the debris. When I pulled my head out of the water, Mike yelled at me to get out, and I did. He pointed to a whitish object near where I had been. "Know what that is?" he asked.
"A dead cat."
FYI, there was a huge amount of large wood in the water that day, but just a small percentage of it was logs. Most of it was dimensional lumber -- some pressure treated with god knows what.