If the user constantly sets their humanoid’s state to swimming, even in a client-based script in a FE game, due to distributed physics, they’ll appear to lean while walking and be able to fly by holding the space bar (since they think they’re swimming). Here’s the code to test with, in a LocalScript:
while wait() do
game.Players.LocalPlayer.Character.Humanoid:ChangeState(4) --> 4 is swimming
end