This update ONLY affects 1.14 users, so if you're not running 1.14 no big rush to update.
Now protects against the swimming villager / merchant exploit. Once a villager's trades are locked in and they start offering a player discounts; if the villager starts swimming or is standing in a water block, EVERY SINGLE TIME the player opens/closes the trade window, the discounts will get better, until the player gets the best possible deal from the trade.
The exploit is showcased about 2:30 seconds in.
Due to a couple limitations/bugs in the spigot API a clean fix is not possible at this time. I have worked out a solution, which removes the water and stops the player's inventory from opening. They will be completely unable to trade with a swimming villager and can't keep increasing the discount. I will improve this once a better solution is found but for now it should keep you safe!