I believe it is Philinorbis acuticauda - it looks similar in the description and the animal was between 5-10 cm long! My husband took that video showing the animal diving into the sand! https://youtu.be/7tAlGzFvKxE

Philinorbis only lives in the deep sea, this animal however was found in less then 20 m. I added a few pictures taken from the video.