You can crawl under blocks if swimming in the correct speed.

*If you swim into a 1 block night area you will crawl (land swim). When you go into 3rd person, you will mostly see black, like in the photo I provided. You will also have the walking particles bellow you

How to reproduce
*make a long 1x1 tunnel
*put on one end a hole in the top
*have water sources next to the hole
*swim into it
*when the water ends, you will keep swimming, aka, Java's crawling not yet correctlly implemented. i couldn't take screenshots sadly *