You can move around in the swimming animation by sticking your head through a ceiling in spectator mode.

By going into spectator mode and sticking your head through a ceiling at least 3 blocks tall and then going back to creative mode, you can move around while floating in the swimming animation. You can fly up and down within the block you started floating in. Double-pressing space or flying down a block will put you back into the standing animation, and switching to survival mode instead of creative mode won't work. This bug isn't harmful or anything, it's quite a funny one.

Here's a link to a video of the bug: https://youtu.be/ED0a_-iMD4s