The bug
Whenever you swim in the new 1.13 snapshots the arms are flipped upside down while swimming. This can create a strange visual that isn't very pleasing on the eye if the players skin has a texture that is supposed to be on one side.
MC-127520
Upside down arms when swimming
Open
7
(Unassigned)
Confirmed
Player Animation
Minecraft 18w11a, Minecraft 1.13-pre6, Minecraft 1.13.1, Minecraft 1.13.2, Minecraft 19w12b, ..., 21w16a, 1.19.4, 23w18a, 1.20.1, 1.21.4
Retrieved
Confirmed for 1.13.1.