In the new beta (1.2.20.1) there is a bug that causes PlayerActionPacket STOP_SWIMMING action (PlayerAction 22) to be spammed from client -> server every tick, even when not swimming.

This has been tested both with a vanilla tracer and a custom server. Both times I see packets with the below content (hex) spammed every single tick:

158770 ms -> Read: PlayerActionPacket - 240000012c00000000

This is an issue like the old InteractPacket bug that will cause performance problems both for the game and for servers.