Nudibranchs of a wide range of sizes are able to crawl upside down using the surface tension of the water. Triopha catalinae, a nudibranch found on the northwestern coast, is the largest nudibranch known to do this behavior (it grows up to 15cm!). Below, you’ll see an illustration I did of this nudibranch along with a photo of the live animal.
Here is another, smaller nudibranch, Dirona albolineata, I collected on a dive near Jackson Beach on San Juan Island a couple weeks ago. Watch carefully and you’ll see this cool behavior in action!