The crops in farm make it so that you can take no knockback, or not even allow hits to be registered. Taking away the crops might help, or making it so these thing don't happen when in the crops.
It's really hard to place a water bucket on someone that keeps moving, especially since the holes in the ground make it so that this is way less effective(water might flow into hole give a single line instead of widespread crop destroying). In the time that you are able to get a hit they could eat and get health regen, this is really unadvantageous if the person you are fighting has better gear than you or you don't have good food to heal with. I have also tried to break the crops under my enemy, but this does not work since they can just keep moving while my sword recharges. Also what if the team is at mid, there's isn't much you can do to force someone to move from the middle island withouth being there in the first place(they can just build a barricade to block the arrows).If this is a problem for you, just place a water bucket on the crop fields before you start pvp. Or you could just avoid fighting other teams in the center island.