Could it be because it's on the server tick side and not client side

Could it be because it's on the server tick side and not client side

Try simply using (please not you will not be able to jump while walking on water due to the nature of setting the Y to 0 )

if(world.getBlock(x, y-1, z).getMaterial() == Material.water){
player.motionY = 0.0D;
}

 

Image(there should atleast be one):

 

 

Thank you so much!  So is there absolutely no way to allow me to jump 

Thank you so much!  So is there absolutely no way to allow me to jump

 

Not jumping as on land but you can add a check to see if the jump button is pressed and if so temporary disable it while you're pressing the jump keybind just replace player.motionY = 0.0D with this(it will also enable you to get onto land again without any problems(i.e. before you would have to find land at the same height w/o it)):

if(!Minecraft.getMinecraft().gameSettings.keyBindJump.getIsKeyPressed()){
player.motionY = 0.0D;
}

So it's also very slow when I walk on water, anyway to speed that up

The other potential method is to override player's collision function and replace collision bounding boxes of certain blocks. Though it might require bytecode manipulation.

I have added this else if statement to let you jump out of the water

			if(flag && e.motionY < 0.0D){
			if(!Minecraft.getMinecraft().gameSettings.keyBindJump.getIsKeyPressed()){
			e.motionY = 0.0D; 
		}else if(Minecraft.getMinecraft().gameSettings.keyBindJump.getIsKeyPressed()){
			e.motionY = 0.5D;
		}

However, now whenever I walk on water with the boots minecraft gives me an error that says "Illegal stance."

 

I think that means that you have a position outside of the world or something like that.  What is your code for walking on water look like now?  I suspect you're adjusting the Y position to some value outside the world.