First do you have the lightbulb from the Yoshi Sphinx? If not you should, so go back and get it then return to the harbor.
Once you've the lightbulb head to the dark room in the harbor house, you'll need to use your lightbulb thing sticker there to brighten the room and then you'll see the ship wheel scrap.
As for how to get into the dark room in the harbor I'm having a memory laps. There are four doors you can take, one is on the back side of the buildings and three are in the front. I know the front left one takes you to just a sticker and I believe the one in the back takes you to the vacuum. I'm almost positive the house you want is the one that had door turned the wrong way, go into paperize mode to fix the door and it should lead you into a building with a stairs that goes downstairs.
The dark room is downstairs, but don't quote me on which door to take to get the stairs cause like I said I'm having a bit of a memory laps on that part.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me