The latest version of the SwimSystem, 2.1.1 is now released on BOOTH
It can be downloaded right here: https://hirabiki.booth.pm/items/2127684
This new version fixed one particular case where when the SwimSystem, initially enabled, has been disabled and a player has never entered into the water since first joining the world, the player will be stuck and unable to move at all until rejoining again.
Thanks to KingxFozzie on discord for finding out this issue!